Question archive

Answer

Light is both a particle and a wave. This is called the particle-wave duality. The best reference that I know of is nobelprize.org.

Answer: Photons are originally generated in the core of the sun from burning hydrogen into helium. It takes them millions of years to get to the surface. They can only travel a very short distance before they run into something (proton or electron) and bounce off. Eventually they bounce around until they get to the surface. Each time they get bounced (scattered) they can loose a little energy to the object they bounce off. So they can start out as high energy gamma rays and end up as light.

Answer: The speed of light in a material is always less then the speed of light in vacuum. The ratio of the speed of light in the vacuum to the speed of light in a medium is called the index of refraction n. It can be show that the frequency of light at the interface between two materials with different indices of refraction for light must be conserved. Consider the wave fronts from a wave of light passing a observer A before the wavefronts arrive at the interface between the two material. Now consider the wave fronts from the wave of light passing an observer B after the wavefronts arrive at the interface between the two material. The time between the the passing wave fronts must be the same for observer A and B otherwise, the wave fronts would pile out at the boundary, be created or destroyed at the boundary (note: the observers must be stationary relative to on another in this example). Since the number of wave fronts must be conserved, the frequency is conserved at the interface(ie the frequency does not change). This can also be shown from the conservation of energy for a photon of light. The energy of an individual photon of light must be conserved at the interface between the air and the glass(conservation of energy). But E=hf (E=energy, h=plank's constant and f is the frequency) for a photon, therefore the frequency of the photon is conserved. Now, for a wave, v=f*lambda where v is the speed of the wave, f is the frequency of the wave and lambda is the wavelength of the wave. If v decreases, and f is constant, lambda (the wavelength of the light) must decrease. So in summary:

1. the speed of light is slower in a materials than in vacuum.
2. the frequency of light does not change between mediums.
3. the wavelength of light is proportional to the speed of the light v=f*lambda ie if v is faster in a medium lambda is longer or if v is slower in a medium lambda is smaller.

Answer: John Dalton proposed that gases are made of atoms (tiny particles) to explain the results from his experiments and to calculate the atomic weight of these gases. However, he did not PROVE that atoms exist. The concept that macroscopic particles are made in microscopic particles, atoms, was first proposed by the ancient Greek philosopher Democritus. In the 1700/early1800s John Dalton proposed that gases were made of atoms.

The proof of atoms came from Rutherford and his student, Geiger and Marsden, proved the existence of the atomic nucleus in 1909 when they scattered finely collimated beam of alpha particles off gold foil. The scattered distribution indicated that a positive charge is is located in a small central region of the atom. This directly implies the existence of the atom. Einstein's paper on "Brownian motion" in 1905 is also credited with the discovery of the atom. Brownian motion is the constant jiggling of pollen gains visible under the microscope. Note, the current idea of the atoms is nothing like the ancient Greek concept of a atom. Water is composed of molecules that consist of one oxygen atom and two hydrogen atoms. When water is cooled below 4C it begins to expand. This is because when water freezes the crystal lattice (hexagonal ice lh) that forms by the ordered arrangement of water molecules is less dense than the random arrangement of water molecules in a liquid. Water also expands when it is heated above 4C (ie 4C to 100C) this is because the water molecules have more kinetic energy to over come the intermolecular forces that hold them together.

Answer: Water is composed of molecules that consist of one oxygen atom and two hydrogen atoms. When water is cooled below 4C it begins to expand. This is because when water freezes the crystal lattice (hexagonal ice lh) that forms by the ordered arrangement of water molecules is less dense than the random arrangement of water molecules in a liquid. Water also expands when it is heated above 4C (ie 4C to 100C) this is because the water molecules have more kinetic energy to over come the intermolecular forces that hold them together.

Answer: Water is composed of molecules that consist of one oxygen atom and two hydrogen atoms. When water is cooled below 4C it begins to expand. This is because when water freezes the crystal lattice (hexagonal ice lh) that forms by the ordered arrangement of water molecules is less dense than the random arrangement of water molecules in a liquid. Water also expands when it is heated above 4C (ie 4C to 100C) this is because the water molecules have more kinetic energy to over come the intermolecular forces that hold them together.

Answer: There are four main types of galaxies: elliptical, spiral, barred spiral and irregular galaxies. Elliptical galaxies are galaxies that have a round or elongated shape as viewed from earth with no visible spiral. Spiral galaxies are galaxies that have spiral arms, similar to that of a whirlpool. Barred spiral galaxies are galaxies that have a bar like structure in the centre with spirals at the outer edge. Irregular galaxies are galaxies that have no symmetry.

When galaxies formed, they grew in size by attracting matter through the gravitational force. Gravity is the same force that pulls you towards the ground. Galaxies can also grow in size if two galaxies collide and merge together. More information and and images cam be found here.

Answer: Humans produce sound by pushing air out of there lungs and past the vocal folds which is between the larynx and the trachea. This causes the vocal folds to vibrate resulting in the formation of pressure waves in the air (sound waves). This frequency is then modified by movement of the tongue, pharynx, palate, jaw, or lips to produce the vowels and other sound that we recognize. A complete description of the process may be found on the links here.

Answer: In astronomy, one observes far away objects (sometimes many millions of light years away). Now light travels at a finite speed (~2.998x10^8m/s2), so the light that comes from an object that is millions of light years takes millions of years to travel to earth. As a result, when one is observes this light they are observing an event that occurred millions of years ago.

Answer: A crystalline solid is a material in which the atoms, molecules or ions have a pattern or ordered arrangement. In contrast, amorphous solids are materials in which the atoms, molecules or ions are arranged randomly. There are four types of crystalline solids: molecular solids(H20,CO2,CH4), covalent networks(C[graphite or diamonds], SiO2), ionic solids(NaCl,KBr) and metallic solids(Na,Zn,Cu,Fe). The intermolecular forces in a molecular solid are the dispersion forces, the dipole-dipole forces (an intermolecular force that result from the electric potential between near by dipoles on molecules) and the hydrogen bond (a bond between a hydrogen atom and an electronegative atom such as O,N or F).

Molecular solids are soft, nonconducting and have a low melting temperature. Covalent networks are formed through covalent bonds, the bond that forms between two atoms that share one or more electrons (generally 2 electrons). This type of crystalline solid is hard and has a high melting point. Ionic solids, materials in which the ionic bonds (the bond formed when one or more electrons are transfered to another atom) hold the ions in a regular three dimensional pattern, are brittle, hard and have a high melting point. Metallic solids, which are formed by metallic bonds, are conductive and have variable hardness and melting points.

Answer: 1,2,6,7 & 8 The constellations cancer, Gemini and Aquarius are three of the zodiac constellations. The zodiac constellations travel across the sun every year as a result of the earths orbit. There are several myths about Gemini (the twins) which can be found here and cancer (the crab) here . Gemini can be seen in the northern hemisphere (this would include Canada) in the winter and early spring while Cancer is visible is the northern hemisphere from December to June. Aquarius is visible from the northern hemisphere in the spring. More information on the constellations can be found here .
5) Historically people have used the stars as a tool for navigation. Groups of stars that were important for navigation were given names giving us the constellations. 3) As of Feb 24, 2005 there are 164 know short period comets (comets that have an orbital period of less than 200 years). For the long period and non-periodic comets, there has been roughly 1000 observed. A table of some famous comets can be found at here . More information on comets may be found here.
4) String theory is a theory proposed by some theorist to describe how the forces and particles in the universe work (It is the only currently know method of combining General Relativity and Quantum mechanics). There is no measurement or experiment to indicate that String theory is correct!! String theory is a description of the forces and particles in nature as vibrations of strings (or membranes) on a 10 (or 11) dimensional space.
9) The idea of multiple universes comes from the need to solve some problem with the theoretical description of our universe. For example, it has been proposed that there is a matter universe and an anti-matter universe to explain why the universe is composed of matter. There is no experimental evidence for the existence multiple universes.
10) Planet-X does not exist. However, there are lots of objects beyond Pluto but they are not being classified as planets. They are Kuiper Belt objects. So far, none of these objects, which have been found, are as big as Pluto.
11) Measurements of the Doppler shift (more info) from light emitted in other galaxies indicates that almost all of the galaxies observed are moving away from earth. Current measurements have indicated that the average speed at which the galaxies are moving away is accelerating. This indicates that the expansion of the universe is accelerating.
12) There is no evidence for the existence of aliens. However, there is no reason to assume that life is unique to earth. NASA has sent several missions to Mars searching for microbial life (ie bacteria) more info.

Answer: A molecule is described as a quantum mechanical system. The vibrational and rotational energies of the molecule are therefore discrete, and the transitions between the energy levels are much smaller then the energy of an ultraviolet photon (3.1eV-124eV). So the interaction will be with the orbitals surrounding the nuclei of the atoms, which is also described with quantum mechanics. Since the transition levels in the molecule are quantized (a fix set of discrete values), the electron can absorbs 1 photon at time. Once an molecule has been excited the excited electron will decay down to a ground state configuration. By conservation of energy, the photon(s) emitted will have equal(less) energy then that of the initial photon which excited the atom.

Since both photons in the gamma ray regime and x-ray regime have more energy, the molecule can not emit them, when it has been excited by one UV photon. Note, the previous statements assumes that the molecule is in a stable configuration and that the energy from the uv photon does not allow the molecule to change into a new configuration that has a lower ground state energy [for example wavelength shifters]. X-ray are normally produced by Bremsstrahlung radiation (radiation caused by high energy electrons being stopped in a material, "Breaking radiation" ) and by k-shell emissions from heavy metals like tungsten. The k-shell (The lowest energy shell of an atom) emissions are created when a high energy electron knocks an electron out of the k-shell in a heavy metal and the other electrons cascade down to fill up the ground state. Gamma-rays are normally produced in nuclear interactions.

Answer: First, let me explain what the scientific method is: The scientific method is a method that enable scientist to construct and accurate representation of how the universe works. In the scientific method there are 4 main steps:
1. Observation and description of a phenomenon or group of phenomena.
2. Formulation of an hypothesis to explain the phenomena. In physics, the hypothesis often takes the form of a causal mechanism or a mathematical relation.
3. Use of the hypothesis to predict the existence of other phenomena, or to predict quantitatively the results of new observations.
4. Performance of experimental tests of the predictions by several independent experimenters and properly performed experiments.
5. Repeat 3 and 4 until there are no discrepancies between data and the hypothesis.
The Big Bang Theory was obtained by the scientific method and is the best scientific theoretical model that describes the origin of the universe.

Answer: The largest three known stars (KW Sagitarii, located 9,800 light years away; V354 Cephei (9, 000 light years away) and KY Cygni (5,200 light-years away) ) have a radius about 1500 times that of the sun. These stars are classified as red super giants. However, these stars (25 times the mass of the sun) are not the most massive. The most massive stars are about 150 times the mass of the sun. More information on this may be found at here.

Answer: Radio-quiet quasars are quasars that do not emit radiation in the radio frequency. About 90% of quasars are radio-quiet. Quasars are objects that emit an enormous amount of energy and are located very very far away (up to 15 billion light years away, new the edge of the visible universe). Current theories indicate that Quasars are a type of active galaxy, a galaxy that has a central region that emits enormous amounts of radiation. A detailed description of quasars and active galaxies can be found at NASA.

Answer: The Elephants Trunk nebula is a emission nebula that has Bok Globules in front of it forming what looks like an elephant trunk. An emission nebula is a nebula that emits light resulting from free electrons combining with protons to form hydrogen atoms. The free electrons and protons are created by ultra-violet light from nearby stars ionizing the hydrogen gas in the nebula. One of the dominate frequencies of light which is emitted when electrons and protons combine to form a hydrogen atom is red light. A Bok Globule is a dark cloud that is composed of hydrogen gas and clumps of carbon and possibly silicates which are less then a micron across. The small clumps of carbon and silicates are commonly referred to as dust. Some images and a little bit of information about the Elephant trunk nebula may be found here.

Answer: There are several reason why scientist suspect that nuclear fusion takes place in the sun and stars.
1) The pressure, due to gravity, inside of the sun and stars is known to be sufficient to produce fusion. The internal pressure is determined from the mass of the sun or star.
2) It has been observed by several experiments that the sun emits a sub-atomic particle called the neutrino. Neutrino's are produced in several types of sub-atomic interactions including fusion.
3) Heavier elements (carbon to iron) have been observed in remnants of exploded stars. These heavy elements are know to be produced by fusion. Also, stars that have formed more recently have a higher concentration of these heavy elements.
4) The stellar model, which is based on the assumption that energy emitted by stars is produced by fusion, have been very successful in explaining the mass luminosity relation (Big stars=Big brightness).
5) The presence of technetium, an element which has no stable isotopes, has been detected in S, M, and N-type stars by means of spectral analysis. This technetium must have been made in the star by fusion since it would have radioactively decayed away if it had been there when the star formed (The length of time from star formation time to observation is estimated with the stellar model).

Answer: Astronomers make lists to keep track of object in the sky. The letter M refers to the list Messier made and the letter B refers the list E. E. Barnard made. Messier's list of nebulous objects was made up at the time of the French Revolution = 1790's and Barnard's is of dark nebula and dates from ~1900. Astronomers now refer to the objects by the number in the list. Therefore, the Horse Head Nebula (B33) is the 33nd object in E. E. Barnard list of dark nebula. There are also allot of other catalogs. SIMBAD is a catalog which lists most of the names of the stars/objects.

Answer: I think the best way to explain this is to start with a single ray of light (a classical view of light). When a single ray of light is reflected off of a point on a surface, the direction of the ray is reflected about the line normal (perpendicular to the tangent) at that point, as seen in figure A and B. One can think of this as a collision in which energy is conserved and the only force on the light ray/particle is normal to the surface. Now lets look at the case when there are more than one light ray. If one looks at two parallel rays of light being reflected off of a flat surface (Figure C), the relative angle between the rays is preserved. This is because the normal lines of the two reflection points are parallel. However, if the two normal lines are not parallel (as seen in Figure D), the relative angle between the light rays is not preserved. This misalignment between light rays reflected at different points on the surface results in the distortion of the reflected image. It is also important to note here that in reflection apparatus such as mirrors which are composed of a glass structure and a reflective surface on the back, the material in front of the reflective material (for example glass) can also cause distortions if it is nonuniform.

Answer: The emission of subatomic particles radiated from black holes, called Hawking Radiation after Steven Hawking who first theorized about its existence, has never been experimentally measured. So, the research conducted into this process is purely a theoretical investigation by theorists who study General Relativity.

In contrast, the state of experimental research into black holes can be illustrated by another interesting question: What experimental evidence is there for the existence of black holes? In fact, there are several observations that indicate the existence of black holes. These observations come from massive objects that orbit a star. (There is a list of some of these objects below). In these observations, the massive object is surrounded by an accretion disk (a disk of matter which is falling towards the massive object). By measuring the period of the star orbiting the massive object, the mass of the object and the star can be determined. The mass of the objects in the list below, are determined to have a mass greater than 3 solar masses (3x the mass of the sun). This is important because the limit on the mass of a neutron star is about 2-2.5 solar masses. If an object has more mass than a neutron star, it is expected to collapse. It is important to note here, that stars can have may times this mass because of the energy generated in fusion which creates a pressure that prevent the star from collapsing. The accretion disk that orbits the massive object emits radiation. This radiation has a quasi-periodic oscillation (an oscillation which has a period that changes over time) resulting from the decaying orbit of the matter in the disk. Eventually this matter falls into the massive object. With the mass of the massive object known, the length of the shortest "period" of the quasi periodic oscillations enables a limit to be set on the maximum radius of the massive object. Moreover, there is no signature (an explosion which produces x-rays) indicating that the matter from the accretion disk has fallen into the massive object. These three observations/measurements indicate that the massive objects are consistent with being black holes.

Answer: A six foot man (1.8m), assuming his feet are at sea level in the boat, sees the horizon at approximately 4.789km or 2.9-3miles. A complete explanation of the method employed to get this result can be found on our web site at: [Answer]. However, if an object is above sea level, the top of the object may still be visible. One can see this by looking at a two object of height h1 and h2. Let h1 be the man, and h2 be the object he is looking at above sea level. The distance at which the second object disappears from the man's sight is when the line of sight between the man and the second object crosses a point on the horizon relative to both the man and the second object (as seen in the figure below). Therefore, a object above the horizon can be seen if the distance(d) between the man and the second object satisfies d < d1+d2= (r+h1)*sin(arccos(r/(h1+r)))+(r+h2)*sin(arccos(r/(h2+r))). For the six foot man, again assuming that his feet are at sea level, is d < 4.789km+(r+h2)*sin(arccos(r/(h2+r))).

Answer: Here is a website that contains the RA and Dec of the brightest stars. You may want to know some more information on the constellations. This can be found here.

Answer: Although some anceint philosphers (Pythagoras (c. 550 B.C.), Boethius (A.D. 480-524), Aristotle (384-322 B.C.), Chrysippus (c. 240 B.C.) and Vetruvius (c. 25 B.C.) ) had desciptions of sound consistent with that of a wave, it is Leonardo Da Vinci (around 1500) who is creidited with the scientific discovery that sound travels in waves. Here is a brief history of the physics of sound.

1. Around 1500, Leonardo Da Vinci discovered that sound travel in waves.

2.In the 1640's, Marin Mersenne made the first experimental measurment of the speed of sound.

3.In the 1660's, Robert Boyle conducted an experiment with a ticking watch in a partially evacuated glass vessel which provided evidence that sound waves travel through a medium, such as air.

4. In the late 1660's, Sir Isaac Newton determined the relationship between the density of a material and the speed of sound in that material. Mainly, that the speed of sound is inversely proportinal to the square root of the density(ρ) of the material [v α (1/ρ)^(1/2)].

5. In the mid 1700's, Daniel Bernoulli explained how a string can vibrate at multiple frequencies.

6. In the 1700's, Euler (1707-1783), d'Alembert (1717-1783) and Lagrange (1736-1813) developed the mathematical and physical concepts required to describe sound in a complete theory. Our modern understanding of sound is a refinement of this theory.

Answer: When light travels through particles smaller then the wavelength of the light, it scatters in a process called Rayleigh scattering. Rayleigh scattering is the dominate scatter process for light when the particles that the light is travels through are roughly 1/10 the wavelength of the light. The amount of scattering is proportional to 1/( wavelength)^4. Blue light has a smaller wavelength then red light, thus blue light scatters more efficiently then red light. This is why the sky appears blue in the day time.

In the evening, the sun is closer to the horizon causing the light to travel a much greater distance in the atmosphere. When light scatters in the atmosphere, some of the scattered light will always be scattered out of the atmosphere. As a result, the further the light travels in the atmosphere the more the density of the light is reduced due to scattering. From this it follow that the density of blue light will be reduced in density more rapidly then red light. When the density of the scattered blue light is significantly reduced relative to the density of scattered red light, the sky appears red.

Answer: The distance between you and the horizon depends upon your altitude above sea level. For a person standing with their eyes at 2m (6 2/3 feet) above the earths surface (or sea), the horizon is approximately 5km (5000m or approximately 3miles) away. The earth's shape is approximately a sphere. Therefore, the furthest point on the earths surface one can see is the point on the earth's surface that is tangent to your line of sight (marked by the point p in the figure below). If r (r=6370km) is the radius of the earth and you are a height h above the surface of the earth, then the angle theta between you and the horizon is theta=arccos(r/(h+r)) as seen from the diagram. Then, the distance to the horizon is (r+h)*sin(theta). For 2m above sea level, a common height in a boat, (6370km+2m)*sin(arccos(6370km/(6370km+2m)))=5050m=~5km=~3miles.

Answer: Acceleration is defined as the rate of change of the velocity of an object (Note: Velocity is defined as the rate of change of the position of an object). The simplest example of acceleration is when a particle changes velocity at a constant rate. Another words, the acceleration is constant. If a particle changes from a velocity v1 to a velocity v2 at a constant rate over a time interval t, the acceleration is (v2-v1)/t.

Answer: There are several methods for determining the radius in your home town. One method is to setup two horizontally separated vertically plumb straight bars, as seen in the image below. Solving the two equations (from the triangles) yields the bottom solution for r. Although this method is good theoretically, experimentally it is not reliable due to the gravitational field of near by object. A more practical method is GPS. GPS uses satellites at a known position to calculate your position. Basically the satellites send out a signal that gives the time that the signal was produced. If you can measure this signal from several different satellites at the same time, you can calculate you current position.

Answer: The difference between ultraviolet and light and visible light is the wavelength. The wavelength in ultraviolet light is smaller than the wavelength of visible light. This causes ultra violet light to be absorbed and scattered in the atmosphere much more than visible light. Therefore ultraviolet telescopes are required to be in space (above the earth's atmosphere). Also, ultraviolet light is absorbed by glass. This means that glass can not be employed in ultraviolet telescopes to focus the light, unlike some optical telescopes which uses glass lenses. Ultraviolet and optical telescopes have may common features. The most important similarity is how they focus the light. They both employ mirrors to focus the light into an image and/or gratings to focus light into a light spectrum. The University of Victoria is associated with an ultraviolet telescope, the FUSE experiment ( http://fuse.pha.jhu.edu/ ).

Answer: First, one must understand what matter is made of. Matter is composed of tiny particles called atoms and molecules (a molecule is a particle composed of atoms). Because these neutral particles are composed of smaller charged particles, they interact weakly through the electromagnetic force. These forces bind the molecules or atoms together and are called intermolecular forces. Materials in which the molecules or atoms that do not have enough energy to leave a relatively fixed position are solids. Materials in which the molecules or atom have enough energy to move around but are not free from intermolecular forces are called liquids. Materials in which the molecules or atom do have enough energy to over come the intermolecular forces are called gases. Therefore, when a water molecules no longer has enough energy to move freely they becomes a solid. For water molecules the solid that is formed is called ice.

Answer: I've taken most of the information that follows from the book 'UV-B Radiation and Ozone Depletion' edited by Manfred Tevini. Ultraviolet (UV) light has shorter wavelengths than visible light which means that it carries more energy. The highest energy UV light has wavelengths smaller than 280 nanometers and is known as UV-C. UV light with wavelengths between 280 and 315 nanometres is called UV-B and UV light with wavelengths between 315 and 400 nanometres is called UV-A.

The interaction between UV light and biological systems is very complex, but the key point is that life (of the type we have here on Earth) is made of organic compounds which absorb UV light. By 'organic', I just mean that the molecules are made up mostly of carbon, hydrogen and oxygen. The wavelength of light that will be absorbed by a particular molecule depends on the properties of the chemical bonds that hold the molecule together. It just happens that the molecules in living cells that contain genetic information (DNA) strongly absorb light with wavelengths around 260 to 280 nanometres (UV-C / UV-B). Other important components of living cells such as proteins and lipids also absorb UV-B light.

The energy from the light absorbed by a molecule may cause the molecule to move or change in a number of different ways. The molecule may vibrate or rotate or chemical bonds within the molecule may break or new bonds may form (called photo-reactions). One of the four major constituents of DNA is called thymine. One of the consequences of shining UV light on DNA is that pairs of molecules of thymine join together to form a 'dimer'. This mechanism is thought to be responsible for much of the damage that UV light does to genetic material.

UV damage to DNA is one of the causes of skin cancer and other degenerative skin diseases in humans and other animals here on Earth. Here are a few web sites with background information on the subject:

• Organic Chemistry
• Photobiology
• Ozone and UV info

It's worth adding that UV light is not always a bad thing. It's thought that about 4 billion years ago interactions between the UV light and the gases in Earth's atmosphere led to the formation of amino acids which are an essential building block for life on this planet.

Answer: Unfortunately there is no one telescope for all objects and for all people. Cost is usually the big factor. The bigger the telescope the more photons it can gather and the brighter the image. The bigger the telescope the bigger the cost. Under a hundred dollars buys a good star atlas which tells you about the stars constellations moon, plantets etc. or get a subscription to Astronomy, Sky News or Sky & Telescope magazines. They come with star maps. Or Join the Royal Astronomical Society.

Generally a few hundred dollars is best spent on good binoculars (7X50) with coated optics. These you can buy downtown at the nature shops. For a thousand dollars get a 10-inch F4.5 Dobsonian telescope or a Celestron 8-inch or Meade 8-inch Schmidt Cassegrain telescope. These are advertised in Sky&Telescope Magazine and Astronomy Magazine, which I think you can get at Bolen's Books. Sky & Telescope magazine can also be found on the web and so can the Royal Astronomical Society of Canada.

Answer: The word 'nuclear' in nuclear energy refers to the nucleus of an atom. Everything (almost!) in the world is made up of atoms. There are 102 types of atom that occur naturally here on Earth. Each atom contains a nucleus made up of particles called protons and neutrons. For example, all hydrogen atoms have a nucleus containing one proton, all helium atoms have two protons, lithium has three protons, etc. The nucleus of each atom is surrounded by a cloud of electrons.

Chemical reactions involve changes in the clouds of electrons around the nucleus. Nuclear reactions involve changes in the nucleus itself. The type of nuclear reaction that produces the power for our nuclear power stations is called 'fission'. Fission is the process in which the nucleus of a heavy atom splits into smaller pieces.

Uranium is chosen as the fuel for most nuclear power stations. When a uranium nucleus splits into smaller nuclei it also releases several neutrons. The neutrons collide with other uranium nuclei and causes them to split apart too. Of course, these uranium nuclei also release neutrons which hit yet more uranium nuclei and so on... This is called a 'chain reaction'. In a nuclear reactor we use some of the neutrons being produced to heat up water (the neutrons are travelling very fast, so we can get energy from them by trying to slow them down). We can use the steam from the hot water to power turbines and produce electricity in just the same way as a regular power station.

You can find some more details at this Berkeley Lab page .

Answer: We have a little information about astronomy careers on our departmental web pages here at UVic. The Herzberg Institute of Astrophysics has a comprehensive page about astronomy careers in Canada. I'd also encourage you to visit the pages of the Canadian Astronomical Society and The Royal Astronomical Society of Canada. You might also want to have a look at the Canadian Space Agency careers page (although a 'space' career is not necessarily the same thing as an 'astronomy' career).

Answer: Well, I think that the very first thing to say is that it probably isn't possible to travel faster than light but we don't know for sure. John Baez at the University of California has a page about this. If we did find some way of travelling faster than light then it's true that we would potentially be able to travel backward in time. However, the early universe was a very inhospitable place so you might not want to go there! Even if it were possible to travel back in time and survive the extreme temperatures of the early universe, there would be one more problem. Before the 'time of last scattering' when the cosmic microwave background was formed, light wasn't propagating freely like it is today; it was only travelling short distances. So, you probably wouldn't be able to see anything at all...

Answer: For your reference, my answer would go something like this: The mass of the moon is approx: 0.73 * 10^23 kg ( NASA planetary factsheet ). The mass of a proton or neutron is approx: 1.7 * 10^-27 kg. The electron weight is negligible by comparison to the weight of a proton so we can ignore it. The moon is approximately electrically neutral, so there is one electron for every proton in the moon. There is approximately one neutron for every proton in an atomic nucleus (in fact there are more neutrons than protons in most nuclei, so this assumption is a bit dubious).

Assuming all of the above, there are approximately: mass of moon / ( mass of proton + mass of neutron )

= 0.73 * 10^23 / ( 2 * 1.7 * 10^-27)
= 0.21 * 10^50 electrons in the moon

That's my answer. What do you get?

PS The above estimate ignores the effect of nuclear binding energy. Ie I've assumed that if I have an atom made up of 'x' protons and 'y' neutrons then the mass of the atomic nucleus is just (x * mass of proton) + (y * mass of neutron). In fact the mass of the nucleus will be a little bit smaller than this, but that's a different question...

Answer: Thanks for your question. That's a very broad topic. Analytical chemists use equipment that ranges in complexity from a simple burette (a glass tube with a tap at one end) to an MRI machine (Magnetic Resonance Imaging). There is a fairly exhaustive list of the techniques used in analytical chemistry at Dr Jack Martin's web site .

More general chemisty links can be found at the websites of the departments of chemisty at the University of Potsdam in Germany and at Liverpool University in the UK. We also have a chemistry department here at the University of Victoria of course.

Answer: You can find lots of information about Helium at Web Elements.

Answer: I'm not an expert in this field, but I don't hold out much hope for finding transparent solar panels. Ths problem is that solar panels need to absorb the light in order to make electricity, so an efficient solar panel stops as much light as possible. It's true that solar panels can only use light which has a particular frequency, so the rest could be let through in principle. However, in practice, I suspect that a piece of semiconducting material that is thin enough to allow any light to pass through it will be very inefficient at producing electricity.

Sorry not to be more positive. If you want to search around on the web to see if you can find anything relevant then I suggest you look for "thin-film semiconductors". I found this introduction to solar cell technology quite interesting. The gory physics details can be found at Durham University if you're interested. There are numerous sites on the web devoted to solar technology (most of them concerned with the environmental implications). Eg. Solar Energy International.

Answer: A convenient way to understand parallax is just to hold an object fairly close to your face (eg hold your finger up about 10cm from the end of your nose) and now alternately open and close your left and right eye. The object (your finger or whatever) seems to move relative to other objects that are far away. That's parallax. As you close one eye and open the other, your viewing point shifts a few centimetres and you see objects that are near to you from a new angle.

Since you know the distance between your eyes and you could measure the angles at which you see the two images of the object, you can use trigonometry to calculate the distance to the object (of course, in the case of your finger, it would be rather pointless to do this).

Okay, so the same effect would occur if two observers were to look at some object out in space from two places far apart on Earth at the same time. They could measure the distance between themselves (easy if they know the Earth's radius) and then measure the angles at which they see the object they're investigating. This method works well, as long as the distance between the observers isn't too much less that the distance to the object.

Now, unfortunately the Sun is too far away from the Earth to use this effect directly to get an accurate measurement of the distance. The Earth's radius just isn't big enough to allow the observers to get far enough apart (there are more details about this at Dr Sten Odenwald's page . So, historically, people have come up with all kinds of innovative ideas to get around this problem. There isn't just one way to measure the distance, there's a lot of different one's... Typically, these methods use parallax to measure the distance from Earth to some other object in the solar system (eg Venus or Mars) and then use extra information they have about that object in order to get the Sun-Earth distance.

Fortunately for us, there's a good web page about this at the European Southerm Observatory (ESO) The project that they are discussing happened back in 1996, so it's too late to be part of it now...but their 'A short history of the sun's parallax' and 'Methods of measurement' sections are still relevant. There's another page with useful diagrams at the Jodrell Bank Observatory in the UK.

Answer: Yes, lunar haloes can display a spectrum of colours. Just like a rainbow, the light is refracted through water droplets. In the case of the lunar halo, the water is in the form of small ice crystals. Different colours (wavelengths) of light move through water at different speeds and this causes them to be bent by different angles. If my understanding of the way that haloes are formed is correct then the inner edge of the halo should have been more red in colour and the outer edge more blue (the opposite of what you'd expect for a normal rainbow).

The reason that lunar haloes are not always colourful is that in addition to the 'refraction' effect there is also a 'diffraction' effect. If the water droplets (or ice crystals) in the clouds are small (of comparable size to the wavelength of the light) then the light 'diffracts' around them and smears out. The result is that you get a white lunar halo (in the case of a rainbow you get a 'fogbow'). So, my guess is that on the night you saw the halo the ice crystals in the clouds were quite large. I've taken pretty much all this information from "Light and Colour in the Open Air" by Minnaert. It was published in the 1940's but it's still a very useful book.

Answer: A light year is the distance travelled by a ray of light in one year. Light travels at a speed of 300 million metres ever second. There are 3600 seconds in each hour and 24 hours in a day and 365 days in a year. If you multiply these together then you'll find that light travels 9,450,000,000,000 kilometres in one year.

Answer: I had to do some research on this one. As far as I can tell, there really wasn't one definite date when scientists realised that nuclear fusion powered the sun. The idea was being considered by many scientists in the 1930's (eg A.S.Eddington). In 1938 Bethe and Critchfield published a paper in Physics Review which proposed that the mechanism powering the sun was the fusion of protons to form helium. Although it wasn't the first paper on the subject, it was the first to give predictions that matched closely to our observations.

Answer: I haven't heard of any specific research in this area, but that doesn't mean that there hasn't been any. You don't say what kind of reactor you mean, but I'm assuming that you're thinking of nuclear reactors. The photoelectric effect is the process where light (either visible light or any other form of electromagnetic wave such as UV light, X-rays or gamma rays) is absorbed by a material which then emits electrons. The energy of the emitted electrons is the same as the energy of the incoming light.

I don't claim to be an expert in this area and I'm not sure how much physics you already know, but here are the things I think would need to be considered before you could put your idea into practise:

i) Just how much energy from a nuclear reaction is emitted in the form of gamma rays? Every time that a uranium nucleus breaks apart (decays), it releases about 200 million electron volts of energy. There are roughly 7 photons released on average each time a uranium nucleus decays and each photon has an average energy of around 1 million electron volts. So, about 3.5 % of energy is released as gamma rays (photons).

ii) Where does the energy from the gamma rays normally go? The gamma rays will interact either inside the reactor or with the shielding around the reactor so that most of their energy is converted into heat.

The idea behind a nuclear reactor is to take all of the high energy particles (mainly neutrons but also including photons/gamma rays) that come from a nuclear reaction and then 'thermalise' them. In other words, the materials in a reactor are chosen specifically in order to turn the kinetic energy of the particles into heat. Heat is essentially just random vibrations of the atoms inside a material. If these materials are good at turning gamma rays into heat then it's bad news for you.

If you use the energy in the gamma rays to produce an electric current, then you're reducing the amount of energy available to heat the reactor. This may be a small effect, but you'd have to do some calculations to find out.

iii) I suspect that the biggest problem is going to be how to harness the electrons produced in order to have a useful current of electricity. Electrons emitted from the surface of a metal by the photoelectric effect will be scattered about and will travel in all directions. There won't be much of a net current, because just as many electrons will go left as will go right. To be useful, all the electrons should be travelling in the same direction. It isn't clear how to accomplish this.

One solution would be to avoid using metals, and instead use a semi-conductor. These are the kinds of material that are used in solar panels. However, these types of technology tend to be expensive and very sensitive to their working conditions (see iv below). Also, semi-conducting materials only absorb light of very specific energies. It may not be possible to make one that would absorb the very energetic gamma rays you're likely to find in a nuclear reactor.

iv) A reactor is a very hot and messy environment with lots of high energy particles moving around so I suspect that your photoelectric material would be damaged over time (assuming it was something more intricate than a chunk of metal). It's the same problem as people like NASA have when building solar panels for use in space. Outside of the Earth's atmosphere, the panels are bombarded with cosmic rays that damage the panels and makes them less efficient at producing an electric current, but more efficient at getting hot....

I hope that this is of some use to you. If you have any more questions or if you've discovered things that I haven't mentioned here, then please let us know.

Answer: Sound waves are pressure waves. They occur when you compress or expand (rarefy) a material (solid, liquid or gas) that then 'springs' back into it's original shape. Have you ever seen one of those 'slinky' springs? Give it a straight shove at one end and you can see a pressure wave travelling along it.

When we speak, we use our vocal chords to alternately compress and rarefy the air. You can imagine a tube of air between you and the person you're speaking to (it's not a real tube, it's just a way of thinking about it). In a compressed region of air inside the tube, the air molecules are close together (high pressure) and will tend to move towards the less dense air around them. In the rarefied regions of air inside the tube, the air molecules are far apart (low pressure) and will tend to be pushed back together by the air around them. The compressed regions become rarefied and the rarefied regions become compressed. The net result is that the wave moves forward. This process happens very quickly. (Here on the surface of the Earth, sound moves through the air around us at about 340 metres per second).

Here are a few links to web pages about sound waves. Some of them are quite complicated, but I hope they help. Let us know if you have any more questions.
http://www.sfu.ca/sca/Manuals/ZAAPf/s/sound_waves.html
http://www.umanitoba.ca/linguistics/russell/138/sec4/acoust1.htm
http://www.sfu.ca/sonic-studio/Sound_Wave.html
http://www.stpaulsschool.org.uk/DJMwebsite/GCSEphys/syllabus_items/hewave/hewaves1.htm

Answer: Firstly, a plasma is a cloud of charged particles. Most plasmas are gases of ionised atoms (ions). Normally atoms don't have an electric charge because they are made up of an equal number of electrons (negative charges) and protons (positive charges). Plasma is actually the most common visible state of matter in the universe. All stars (including the sun) are made of plasmas. The atoms in the star are at a very high temperature (millions of degrees Kelvin) which means that they collide with one another very fast (the kinetic energy of the atoms in a simple gas is proportional to the temperature). These collisions knock electrons off of the atoms and leave them ionised.

Although the plasmas in stars are very hot, they are only dense near the core of the star where the gravitational field is very strong. In the outer regions of the star the ions tend to spread out. The density of the plasma in the corona of the sun is about 10^18 times less than the density of atoms in a typical gas here on Earth. This means that if you could fill a bottle with plasma from the sun's corona then it wouldn't feel hot, even though the individual ions in the bottle might be moving very fast. There just wouldn't be enough ions in the bottle to heat it up.

Physicists at institutes such as ITER are trying to recreate the conditions present in the sun that allow hydrogen atoms to fuse together to form helium. This involves containing a hot plasma of ionised hydrogen inside a magnetic field.

Plasmas are also used in industry (eg to toughen the metal used in cars and aircraft) and in everyday products (eg fluorescent lamps).

Have I mentioned eight physical properties yet? You can probably find more if you follow the hypertext links in this mail.

Here's one last fact about plasmas (you can decide if it's special or not). Plasmas can block electromagnetic waves (eg visible light, ultra-violet light, infra-red light, x-rays, etc...), but only if the frequency of the wave hitting the plasma is less than the 'plasma frequency'. When the wave enters the plasma it causes the charged particles to move. If the particles can move quickly compared to the frequency of the wave then the wave can't get through the plasma. It's as if the charged particles keep moving in the way.

However, if the wave is oscillating very fast (high frequency) then the charged particles can't move fast enough to 'dampen' it. The wave moves through the plasma and comes out the other side. In some ways, metals are like plasmas (because they have a large cloud of electrons). We all know that visible light is reflected by metals, otherwise they'd be transparent like glass. However, experiments have been carried out with some metals (eg sodium) that show that ultra-violet light (a slightly higher frequency type of light than violet light) can shine right through the metal. I think this is pretty neat, so I hope you do too.

Answer: Are you planning a holiday? Remember to pack lots of water because it's going to be a long trip and Venus is very hot. You can find out lots of facts about Venus at NASA's venuspage. The distance between the Earth and Venus changes with time because they are orbiting the sun at different speeds. The minimum distance between the Earth and Venus is 38.2 million kilometres but the maximum is almost seven times that. So the amount of time that it would take to get to Venus from Earth will depend on when you leave and what route you take.

There's never been a manned space mission to Venus, but there have been lots of unmanned probes such as Pioneer, Mariner and Magellan. This NASA page will tell you how long the Magellan probe took to reach Venus. It would probably take much longer to send a spacecraft with astronauts in it (if you accelerate the spacecraft too quickly then you will injur the astronauts).

Answer: I don't know for sure which theory you mean because there are lots of different ones. First let me tell you about some of the experiments that have been carried out recently.

The first thing to know is that the speed of light is a constant in a vacuum (299792458 metres per second exactly). A 'vacuum' just means empty space without anything in it at all. However, when light is moving through a 'medium' (water, glass, etc) its speed is reduced. The higher the 'refractive index' of the medium, the slower the light moves. There were some experiments done in 1999 where light was slowed down to just 17 metres per second when is passed through a gas of sodium atoms. Other experiments carried out earlier this year slowed light down so much that it stopped completely ( Physics World ). Now anyone can move faster than light!

Of course, when most people talk about the speed of light they mean the speed of light in a vacuum. So, can anything go faster than that? Well, last year there were some experiments involving a phenomenon called 'gain-assisted superluminal light propagation'. Sounds impressive eh? This just means that a pulse of light was made to go faster than it would do if it were in a vacuum. I think these may be the experiments that you are asking about. This subject is very complicated and I am not an expert in this field but I'll try to give a simple explanation.

In one experiment a pulse of light was emitted by a laser and passed through a chamber full of caesium gas that had been specially prepared by putting energy into it. The pulse of light from the laser seemed to appear on the far side of the chamber before it had even arrived at the near side! This excited a lot of scientists as well as a lot of people in the media. However, it turns out that this phenomenon is fully explained by our current theories and doesn't require any new ones. No laws of physics have been broken and unforunately, no, we can't travel back in time using this process.

Firstly, we must remember that light is a wave. This means that a pulse of light is typically spread out over a long distance. It has a 'peak' where most of the light is, but also long 'tails' which don't contain much energy. Long before the peak of the pulse of light reached the chamber, the long tail ahead of the peak hit it. This means that the chamber 'knew' that the pulse is coming. The energy needed to create the pulse of light that was emitted from the far side of the chamber came from the energy that was put into the caesium gas before the laser was turned on.

So, something did move faster than the speed of light (the peak of the light pulse), but the energy in the pulse didn't. This is the important point. Sometimes 'things' do travel faster than the speed of light, but energy doesn't.

In order to be able to travel back in time we would need to be able to send energy (ourselves) faster then the speed of light. No one has ever done this and it's probably impossible. Travelling faster than light doesn't mean that you must go back in time but it means that you could if you wanted to. If you want to read some more about this experiment then you can look at this Physics World article. Even though it can't be used for travelling back in time, it's still a very interesting experiment that may have lots of important applications for future technology.

There are lots of other interesting possibilities for faster-than-light travel that have been thought of by theoretical physcists. One is the famous 'warp drive' that appears in Star Trek. Although this is science-fiction it is based on a genuine principle of Einstein's theory of general relativity that allows space and time to be bent or warped. All matter warps space and time but a warp drive would require the use of particles with special properties that no one has discovered yet. They may not exist at all. We even have a name for particles that travel faster than light. They are called 'tachyons'. Again, no one has ever detected them and they probably don't exist.

Answer: Thanks for your question. I can't give you an exact answer of course. Everyone will have a different number of electrons in their body and there are far too many electrons for anyone to count them precisely anyway. I'll give you my best estimate.

I'm not a biologist, so to find out how much of each element is in the human body I've relied on the answer to a question at the MadSci question/answer site (MadSci).

Okay, so let's say that an average person weighs 70kg (kilograms). This is mostly made up of the following elements (types of atoms):

45.5 kg of Oxygen
13.0 kg of Carbon
6.7 kg of Hydrogen
2.2 kg of Nitrogen
1.1 kg of Calcium
0.7 kg of Phosphorus

Each atom of a particular element has a specific number of electrons:
8 electrons in Oxygen
6 electrons in Carbon
1 electrons in Hydrogen
7 electrons in Nitrogen
20 electrons in Calcium
15 electrons in Phosphorus

So, the next question is, how many oxygen atoms are there in 45.5kg? Well, Avogadro's number is defined as the number of carbon atoms in 12g. It's about 6*10^23 (six hundred thousand billion billion). Carbon has an atomic mass of twelve atomic mass units and oxygen has an atomic mass of about sixteen atomic mass units (because a carbon atom has six protons and six neutrons in its nucleus and oxygen has eight of each). Putting that all together we find that there are:
45.5 kg * 1000 g/kg / 12g * (12 a.m.u. / 16 a.m.u. ) * (6*10^23)
= 1.7 * 10^27 oxygen atoms

Okay, so I've repeated this for all the different elements and multiplied by the number of electrons in each atom to get the numbers below:
1.4 * 10^28 electrons from Oxygen atoms
3.9 * 10^27 electrons from Carbon atoms
4.0 * 10^27 electrons from Hydrogen atoms
6.6 * 10^26 electrons from Nitrogren atoms
3.3 * 10^26 electrons from Calcium atoms
2.1 * 10^26 electrons from Phosphorus atoms

Well, that gives us about 2.3 * 10^28 electrons in the human body (twenty three billion billion billion electrons).

Answer: It is called a lunar halo ( there are solar halos too, but, although they occur quite frequently, they're often less obvious). You can find a good picture of a lunar halo here and a solar halo here .

As those web pages will tell you, it's a refraction effect. The light reflecting from the moon's surface passes through a region of cirro-stratus cloud. Cirro-stratus clouds are high altitude clouds composed of small ice crystals. The light from the moon is bent as it passes through the crystals in exactly the same way that sunlight bends when it passes through raindrops to form a rainbow. The amount of bend is determined by the refractive index of the crystals. Water/ice has a refractive index around 1.3.

(When rainbows are formed, the light is refracted once as it enters a raindrop then it is internally reflected off of the back of the raindrop and finally it is refracted again as it leaves the raindrop. In lunar halos the reflection step doesn't occur; the light is refracted as it enters the ice crystal and then is refracted again as it leaves the crystal.)

The angle between the moon and the halo depends upon the shape of the ice crystals as well as their refractive index. This is because prisms of different shapes have different critical angles below which they cannot refract the light. Lunar halos are usually observed to be at an angle of 22 degrees from the moon. If you carry out some geometry then you can show that this is the critical angle for a hexagonal prism with a refractive index of 1.3. So, the ice crystals in cirro-stratus clouds must be shaped like hexagons.

I'm not a meteorologist, but it does seem that halos do often precede weather changes. You can find a brief explanation here. Please let us know if you see any more interesting atmospheric effects.

Answer: In the mid 19th Century Europe, James-Clerk Maxwell and Ludwig Boltzmann independently derived the form of the Maxwell-Boltzmann distribution. They were both working on the 'kinetic theory of gases'. In other words they were trying to describe how the motion of individual molecules in a gas was related to the quantities we can measure in the lab, such as the temperature of the gas.

The molecules in a gas are not all travelling with the same speed. The Maxwell-Boltzmann distribution predicts the number of molecules in the gas that will have a certain speed (or kinetic energy). (Technically, the distribution applies to any collection of weakly-interacting 'classical' particles that are in thermal equilibrium with a heat-bath....)

The problem with the Maxwell-Boltzmann distribution is that it only works for 'classical' particles (such as molecules or atoms of a gas at room temperature). It turns out that it doesn't work when quantum mechanical effects become important... In quantum mechanics, we can divide particles into two different categories: fermions and bosons. Fermions (eg electrons, protons, neutrons) obey a set of rules called 'Fermi-Dirac Statistics' and bosons (eg photons of light ) obey a set of rules called 'Bose-Einstein Statistics'. The theory behind this is pretty complicated, but the effect is:
i) You can't put two identical fermions in the same 'place' at the same time.
ii) Bosons actually 'prefer' being in the same 'place' at the same time.

Okay, so what does quantum mechanics have to do with gases? Normally, the answer is that you can safetly ignore quantum mechanics. This is because there are so many different ways you can arrange the gas molecules (degrees of freedom) that they will hardly ever try to be in the same place at the same time. However, if you carefully cool the gas down so that the molecules have very little energy then they will collapse down into a smaller and smaller region of space. So, for cold gases you will have to modify Maxwell-Boltzmann's distribution by using either rule i) or rule ii) depending on whether the gas molecules / atoms are bosons or fermions. (Technically, this involves calculating something called the 'partition function'....)

There are lots of interesting experiments being carried out at the moment where gases of bosons are cooled down to form something called a Bose-Einstein Condensate.

Answer: The index of refraction (or 'refractive index') of a material is defined to be the ratio between the speed of light in a vacuum and the speed of light in the material. So, by definition, the refractive index for a vacuum is exactly 1.0. The refractive index of air is around 1.0003 (the exact value will depend upon temperature, pressure, etc.)

Although the refractive index of our atmosphere is very close to the refractive index of space, the difference is large enough to give lensing effects. Water vapour in the air increases the lensing effect a lot, because water has a refractive index around 1.3

Answer: The short answer is that we don't know for sure. It's really important here to make a distinction between what cosmologists and astronomers know from direct observation and what is predicted by our current theories of cosmology. Are you familiar with the idea that when we observe objects in space (either using our eyes, or using optical or radio telescopes) we are actually looking 'back in time'? This is just because light takes a finite time to travel. When we look at the sun (not something you should actually do without special eye protection!), we see it as it was eight minutes ago, because that's how long light takes to travel to the Earth from the sun. So, when we use powerful telescopes to look at very distant objects we are actually seeing objects as they looked billions of years ago.

Unfortunately, we aren't able to look back to the very beginning of the universe. The very oldest image that we can see is the 'cosmic microwave background' (CMB). We think that the CMB consists of light that started travelling a few hundred thousand years after the big bang. Why is there no light older than that still in the universe? Well, the CMB is thought to be an image of a period called the 'epoch of last scattering'. This was the period of time when the charged particles in the universe became cold enough that they could join together to form neutral objects (such as atoms). Before this time, all the matter in the universe would have been a plasma (a soup of charged particles) and light couldn't travel very far before it would collide and scatter from one of these particles.

Prof Douglas Scott at the University of British Columbia gives a useful explanation of the CMB on his Frequently Asked Questions page. Okay, so we can't 'see' what happened in the universe for the first few hundred thousand years of its existence. However, what we can do is try to recreate the conditions of the early universe in our laboratories. This is part of what particle physicists (like me) try to do. Essentially, we accelerate beams of particles (usually electrons or protons) to very high energies and then collide the beam into something (either another beam of particles, or else a stationary target). Why does this tell us anything about the early universe? Well, according to all our theories, the early universe must have been a very hot place, and all the particles would have been interacting at very high energies, just like the ones in our accelerators and colliders.

There are lots of particle physics experiments being carried out around the world. One of the most relevant to the early universe is probably RHIC at Brookhaven in the US. However, important work is done here in Canada at TRIUMF and at CERN in Switzerland.

Answer: Here is a picture of the closest black hole candidate. They find these by their x-ray emissions. NASA has a collection of simulated pictures. They are formed by a collapse of a massive star. The math is not too bad now that we have maple (computers), but the general relativity takes a while to understand. Reality is not what we are used to in these high acceleration environments. The black hole loses energy via Hawking Radiation UK site and a library site.

Answer: This is a good question. There are no magicl numbers to quote to you. In our own solar system there are examples of 3 planets that are in a location to support life, namely Venus, Earth and Mars. Other considerations come into the equation, such as Mars being a smaller planet. Its gravity was not enough to hold on to most of its atmosphere, making it a poor candidate to support life now, but in the past it may have been a great place. This 'life zone' around a star would certainly vary depending upon the mass of the central star. The more energy eminating from the star, the further out the 'life zone' would extend.

There are examples in our own solar system of a possible location for a life sustaining environment that is not in what we would consider an ideal distance from the sun. Io, a moon of Jupiter may have the conditions necessary for life. This is a special case due to its volcanic activity and probable water concentration. It has a problem due to its high radiation levels though. If it's not one thing its another.

Finally, what kind of life do you invision? For human life, 300,000 km may be the best distance. What about other forms of life that might require more or less radiation or higher or lower temperatures? So, there are many other considerations, as well as distance, to define the life sustaining environment and answer your question.

Answer: It takes force to move an object throught a media like air. While the acceleration due to gravity is the same for both objects, the force is different (since force = mass x acceleration, and the mass of the book is more than that of a single page).

The heavier the object, the closer the rate of fall approaches what would happen if there were no air. Since the paper has a large surface area relative to its weight, it is a good example of the air resistance slowing the fall of the object. When you get to a book, the air resistance is much less of a factor. It would be hard to time the difference in rate of fall between a book and a lead slab of the same size. They both fall close to the rate that they would if there was no air. What would happen if you could keep making the page lighter? What if the page was lighter than air?

Answer: I am not quite sure what you are asking. The dimension of volume and area are not the same, so you can not do straight math on the two together. Calculus will give you the answer for either of the two, though. If you must combine the two, then just forget about the surface area, as it has ZERO volume, and figure out the volume.

Answer: The wave particle duality says the electron IS a wave for part of the time. It can be sent 15 20 feet (or 3 to 5 meters to me), but the probability that this electron could tunnel that distance is so small that you would have to wait until the universe has rotated several times before it is likely to have occurred.

The whole atom is even harder, because it is so much larger and is composed of many electrons and quarks. All of which have more of a probability of tunneling out of the atom than the atom does of tunneling accross a room. Just think about how stable atoms are, you would not expect nuclear reactions to be going on to any large extent in your body, which would be the case if the constituent quarks and electrons had a large probability of tunneling out of the atoms of your body. The atoms in your body tunneling out is far, far, far less likely than even that. Our problem is that we can not do something that will make the probabilities improve. When we do experiments to see the tunneling, we change the probabilities by changing the height or the depth of the barrier that the particle must tunnel through. The way to do this for a person and a jar is to put the jar right up against the person and make the walls less than an atom thin, then we still have the problem that the subatomic particles of the person are tied up in the atoms. Another problem is directing the particle. It is just as likely to go in another direction. In fact it is more likely because there is not the walls of the jar to tunnel through, just the air.

Answer: The event horizon is a sphere surrounding a black hole singularity (point like) that varies in radius depending upon the total mass contained in the black hole (more mass = larger radius sphere). This sphere is not made of a physical material. It is special beacause of the properties of space in this region. What the black hole in the centre does is warp space in the vicinity. Out where we are (far away from any black holes), we do not notice anything (except for the gravity we feel from the Earth, which is warping space where we are). As you approach a black hole, the distortion of space is increased and you feel a stronger and stronger force. Since we are talking general relativity here, there is a time dilation that comes into the works too. You may remember that as you accelerate (like we all are on Earth, acceleration due to gravity ~ 9.8 m/s^2), the rate of passage of time slows down for you relative to an unaccelerated observer. Light feels the same effect that we do. The special thing about the event horizon is that the acceleration (and therefore the time dilation on the surface of the sphere) is so great that even light is slowed down to nothing (v=dt, and t is stopped, so therefore v=0 too). Since nothing may travel faster than the speed of light, at or beyond this region, nothing may escape the black hole.

There is another action going on that permits the size of black holes to decrease, even though nothing can escape them. Hawking radiation permits the loss of energy from a blasck hole, thus leading to a reduction of the mass (mass=energy via e=mc^2) inside the event horizon. This is like the black hole absorbs negative energy just on its surface and allows the positive energy counterpart to escape. It has to do with the vacuum exitation energy. I can go into this further if there is interest.

Answer: This sounds suspiciously homework-ish...but OK. Newton told me F=GmM/r^2=ma. If we cancel the m on the right part of the equality we get a=GM/r^2, where a is the 'g' you asked about and G is the gravitational constant and M is the mass of the Earth (another constant) and r is the radius to m (which is 2* radius of the Earth in this case, one radius-of-the-Earth from the centre to the surface and one from the surface to the m). Plug in the numbers (do it for r=Earth surface to check, you should get ~9.8m/s^2). I get about two and a half m/s^2 for your question.

Answer: There is a weird thing that is a result of quantum mechanics called wave particle duality . It really means that in fact things like electrons have a wave nature and a particle nature. It is possible to do experiments where you see electrons as waves (ie you see interferrence effects that are the result of waves canceling and reinforcing). To transfer a persons atom to a jar is OK, as long as you use conventional means. To beam the atom into the jar on some wave is not possible for us. There is an effect called tunneling that does something like this, but its effects are only at a very very small scale (the size of an atom). This effect has a certain probability of happening. The probability gos to an extremely small number as you move away from the source and as the distance to tunnel increases much greater than the diameter of a nucleus (of an atom).

Answer: From personal experience, there are a few reasons I believe the Earth is a big sphere (or at least close to a sphere -we all know it is not a perfect sphere)

1. When I went to Mexico, the days were a different length of time.
2. When I went to Saskatchewan, the days started at a different time.
3. When I went from Calgary to Saskatoon (many times), the mountains got smaller in height until they went below the horizon.
4. When I went from Colorado to Saskatoon (only once), the mountains got smaller in height until they went below the horizon.
5. There are tides here in Victoria where I live. Hard to explain without spherical Earth theories.
6. The sky is blue during the day and redish at sunset. This is due to the scattering effect of the rays of light from the sun and the distance the rays travel through the atmosphere. The blue end of the spectra is more likely to be scattered than the red. Thus the sky looks blue during the day because you are seeing the blue rays that are scattered through the short distance from the upper atmosphere to the ground. The sky is redish at sunset because you are seeing the red end of the spectrum getting through, while most of the blue has been scattered out (where it is day still). So the distance throught the atmosphere must be longer at sunset -> spherical Earth.
7. The angles work out. I went to Europe once. I did not actually measure this, but I could have. The sum of the inside angles of a triangle are different on a sphere than on a flat surface. The angles I observed going Toronto->UK, UK->France, and France->Toronto could have been measured to verify the sphere theory. You can do this at home if you have a car, lots of gas and sandwiches, a straight set of roads and few hours to kill. Drive 100km one direction, turn right (measure the angle), drive 100km, turn right so that you are pointed at your starting point (measure the angle), drive home. The sum of the angles will NOT be 2 pi. If you measured the final leg distance, you may be able to get an idea of the curvature too. For instance, if you start at the North pole and go in any direction until you hit the equator, then turn 90 degrees (so you are going along the equator) and go 1/4 aaround the world and turn 90 degrees towards the N pole again. When you get to the north pole (and you will) you will find the angle between your starting path and your finishing path is 90 degrees. Thats 3 90 degree angles in a triangle! (On a flat surface you can only fit 2)

Answer: Deep sea divers must remember to exhale constantly when rising to the surface from any reasonable depth. To fail to do so would result in the diver's lungs exploding. I do not think there would be any difference in the case of a person going into a vacuum (ie outter space). So the key may be to exhale and get as much pressure out of your lungs as possible. One thing that could save your lungs is the pressure change is not that much (ie 1 atmosphere -> 0 as opposed to many atmospheres -> 1 in the case of the diver). It takes your lungs about .2 seconds to react to the pressure change, so just how are you going to go into the vacuum? If the decompression is faster than .2 seconds, you are done for. More importantly, how fast can you get out of it and back into a pressurized cabin? The previous link states that you have about 9 - 12 sec. of useful consciousness if you survive the decompression. That is decompression to an altitude of 40,000 ft (~13km), not a total vacuum.

This is ignoring the effect the vacuum would have on the rest of your body. There is a research institute devoted to this sort of thing. Mostly with space suits, though. Weightlessness is the health factor in this case. I will keep looking for any definitive zero pressure effects that could occurr in the ~9sec. you have.

Answer: For the first part of your question (putting matter on a light wave), the answer is no. Light moves at the speed of light, and nothing with mass can travel at the speed of light (it would take infinite energy to accelerate it to that speed). Even if the speed issue were not a problem, another problem would be that light waves interact with matte . This is why you can see (light waves being absorbed by your retina) and why you can see (light waves being given off by the tungsten atoms in your light bulb).

Your question could be interpreted differently, though. Since we know it is possible to put matter on matter waves (ie logs moving along on the ocean, loggs=matter, ocean=waves), and since matter rests on matter (rather than moving through it) on residual coloumb force which is an electromagnetic force which (according to particle physics) is a force mediated by photons (photons=light), in a way it is possible to put matter on light waves, that is what happens all of the time. I hope I have confused you enough.

For the second part of your question (atomic fusion and the jar), the answer is - what? First off, ripping atoms apart is fission, not fusion. Fission = cando reactor, fusion = sun. We mere mortals are not yet adept at obtaining a net energy gain from fusion (except in the natural sense).

To put someones atoms in a jar would not work since when you rip apart the atom, you are left with energy yes, but also with atoms (lighter ones in the case of fission, heavier ones in the case of fusion). Maybe you are thinking of ripping apart the atoms in the chemistry sense (ie taking a molecule and reducing it to atoms). You may get energy from doing this too, but it is chemical energy (like when you burn wood in a fire and get heat) and you are left with the same atoms in the end, only forming different molecules than when you started.

In either case, it would be hard to get the persons atoms into the jar after the fusion, fission, or chemical breakup. You do not save a lot of space in these processes, although the net volume at STP would vary depending upon the final state.

Answer: Yu need to 'be good at' your courses enough to get the minimum GPA required to get in to the university of your choice. This varies depending upon the institution. Some university departments require a GPA at or above the mid-70's for admission.

Now there is the issue of the courses to take to prepare you for your future in astronomy. I can only speak from what I know, which is for admission to the University of Victoria. In this case, the university has a requirement of the fllowing courses...

• one of Francais 11, French 11, German 11, Japanese 11,Latin 11, Mandarin 11, Spanish 11
• satisfactory completion of Chemistry 11, English 11, Mathematics 11, Physics 11
• English 12, Mathematics 12 and two of Biology 12, Chemistry 12, Computer Science 12, Geography 12, Geology 12, Physics 12

The 'satisfactory completion' part is the GPA clincher. You should note that some universities have quotas for out of province students, so you may need a higher GPA if you are planning to go to a university outside of your home province.

Here are some information links for you if you would like to come to the University of Victoria

• Science Admission requirements
• UVPhys Online Academics Page
• Admissions Page

If you know you want to come here, here are the links to get the forms...

• Admission request forms
• APPLY ONLINE
• Calendar
• Academic Advising
• Web Reg

The way to do it is to apply, then wait to get your admission approval. Then use web reg to get into your courses. Do the web reg as soon as you are able to (you get a date on your approval form), since many 1st year courses fill up. While you are waiting for the approval, you may use the Calendar and Advising links to see what courses you should be taking. That way, you are ready to register (via web reg) as soon as your date comes up.

There may be a deposit required before you are able to use web reg. It will tell you on the form. I think they still send out paper webreg booklets with the registration package. The numbers for the courses are what you enter on the webreg web page.

Answer: I have looked at the information available. It is harder to make a marimba than it would at first appear. I could give you an answer based on physics alone, but I feel it is safer to ask someone who has more experience in the creation of the instrument. Here are some links (not endorsements).

• The scientific approach with numbers
• A brief intro on construction
• Virtual Museum of the Mexican marimba
• History
• A page of links

A long delayed response - I have been away for a few weeks. Here is some info for you sound buffs.

Answer: I am working on it, this one is outside of my area, so I will need some help. I do know that it is all about waves. This leads me to an initial conclusion that sharp bends in a square tube would allow for resonant frequencies other than the ones the fellow is seeking. Straight, round, tubes or curved tubes (without sharp kinks) should be better in this case. I think one of the problems with square section tubes would be in getting the system aligned so that the blocks can set up the reverberations. I would imagine the same is true in hearing the reverberations, with the sound being directed along the flat faces of the tube. As an aside, the pipes do not have to be closed at one end to resonate. I found this facinating when I was first learning about sound.

Answer: I have found this listing of US Physics Departments. You should take the list with a grain of salt, since any ranking of this sort is fairly subjective.

There are a bunch of data tables you can look at at the US National Academy of Sciences site.

The area of physics you are interested in will be a really big factor in the university that comes first. This is doubly true if you are thinking of starting a career in theoretical physics. I think it would be best to contact an advisor in your local university. My perspective is from Canada, the two systems are a little different and I do not want to point you in the wrong direction.

It is usually not to difficult to go and talk to a professor. You can pick their brains about what to do and where to go. Feel free to reply if you would like to discuss this further. What area of physics are you interested in? Re-reading the question, I see that you may just be asking for interest sake and not looking for a place to go to school. Since the previous two questions were application related, I am on that track. In that case, the NAS files will be the long answer to your question.

Answer: The acceleration due to gravity on Earth is about 9.8 m/s². This will vary only slightly depending upon your geographical location. It is even pretty good for different altitudes. That is why you do not feel lighter when you are on an airplane (at least most of the time). Your results may vary if you decide to take a space shuttle ride, since the altitude is enough to change the value considerably. Even then, gravity is strong enough to keep the ship in orbit around the Earth.

The acceleration is based upon Newton's formula for gravity: F=GMm/r². Since Newton also gave us F=ma, we can combine the equations and cancel an m and get: a=GM/r². In these equations, F=force, a=acceleration, m=your mass, M=the Earth's mass, r=the distance to the centre of the earth, and G=the gravitational constant. Notice that G and M are constant, and r is a number that will change little (in percent) on the surface of the Earth (since the distace to the centre of the Earth is so much greater than the minor deformations in its surface we call mountains).

There is a dirty little secret hiding in the equation. One of the initial assumptions for this equation to hold is that the Earth must be a perfect sphere with its mass distributed evenly. The person experiencing the force must be one of these perfect spheres too. Why is this seldom mentioned? Because it does not matter. Relative to the Earth, you and I are just points, and points are perfect spheres of a sort. The mass distribution and the geometry of the Earth are close enough so that the perfect sphere approximation works.

So, the acceleration due to gravity does change slightly depending upon your location on earth, but the above number is pretty good. As an example, if you were at an altitude of 1000km (about 10x the space shuttle altitude) the acceleration due to gravity would be 7.3 m/s². Still substantial. The Handbook of Chemistry and Physics (CRC press) has a listing of more specific numbers if you need them. Where does Einstein figure into all of this? I am glad you asked. Since we are slow compared to light, and the gravitational field of the Earth is fairly small, the correction to Newton's equation are negligible.

Answer: This is an easy one, as long as you know if you are laying the spoons end-to-end, side-by-side, or stacking them. I will assume side-by-side, since that is the compromise. For a common stainless teaspoon I found lying around, there would be... 40,258,427,540,000,000 +- 5,000,000 teaspoons/ lightyear (side-by-side).

I will have to break out the double precision on my computer if you want to reduce the uncertainty from +- 5 million.

Answer: This is a fairly specific question. This sounds like a project specification or something. I think you already have something like this, yourself. If you make up your mind to work on something a minimum of 5 times, then you will transfer the energy from yourself to the object as you require. You may respond with more specifications and I will try to be more helpfull.

Answer: Actually, The number of days taking data is quite small compared to the analysis. Most data is now collected using a CCD and not a human observer. In principle, the modern astronomer does not even have to be close to the observatory that she/he is using (take the Hubble for instance). The practice is a little different. Astronomers still get up in the middle of the night to point and check their telescope when they are taking data. This is just for observational astronomers. There are radio astronomers and theoretical astronomers as well. Many modern astronomers do not even use telescopes at all, they instead create computer simulations. By comparing the results to observations, these simulations (which look like really cool video games to me) can assist the scientist in determining the validity of the theories that were used in creating the model.

So it really depends upon which branch of astronomy you are in. If you are looking for near earth asteroids, you would probably be doing nightly observations (or at least the computer you use would). If you are doing modelling of the creation of the universe, you may never go near a telescope. Astronomy is a very diverse field. Here are some of the results.

Answer: The most elementary particle known to date are in fact particleS. The quark being one of them. There are in fact 6 types of quarks. The quarks, along with another elementary particle, the electron, make up the matter which we see. Two up and one down quark make a proton and two down and one up make a neutron. All evidence points to the quarks and electron being 'elementary' (ie, they are indivisible point-like particles).

There are also particles that are like electrons, these are the Muon ans Tau particles. Associated with the electron, muon and tau are the neutrinos (one each).

The final type of elementary particles are the 'Bosons' which mediate the forces of nature. These include the familiar photon, as well as the W+, W-, and Z0 which are associated with the weak nuclear force, and the gluon which is associated with the strong nuclear force. The graviton , which would presumably mediate the gravitational force, has yet to be discovered.

If you count the anti-particles , the total comes to 34 (which is short of the Hitchhiker's Guide to the galaxy claim of 42). They are all classified according to their properties. There is a fury of activity surrounding the search for a new particle dubbed the Higgs . This particle is thought to give rise to the mass of objects. There could be more than one Higgs particle, but 42 still looks high. Do not be confused by the multitude of particles in books on particle physics. These particles are made up of combinations of quarks, much like the proton and the neutron.

Answer: The probem with the gravity waves is two fold...Firstly, they are so hard to detect that they have never been observed . Even those thought to arise from black holes orbiting each other (I would hate to see the size of the phone that uses two orbiting black holes as the transmitter).

Secondly, they still only propagate at the speed of light. If you think about this, it makes sense. Why should gravity be any different than anything else in this respect. How about travelling faster so the communicator does not have to be so complex?

Answer: The branch of physics that studies the creation of the universe is called Cosmology. The famous Big Bang theory is the leading candidate for the origin of the universe. This is not because it is 100 percent correct (it is known to have flaws) it is because it successfully explains the shape of the cosmic microwave background spectrum and the origin of the light elements. Cosmologists have extended the original Big Bang theory to what is known as Inflationary Big Bang theory. This was done to better fit the theory to the observations of the size of the universe, the uniformity of the universe in different directions, and the perterbations that created matter.

Answer: The star is not quite a massive rock, since a rock is a solid and a star is made up of gas, plazma and, in some cases, dissociated atoms(a neutron star). The explosion of a nova or supernova comes not from the "rocks" but as a shock wave from the collapse of the shell of the star when it can no longer be supported by the fusion process. Remember the scale of things we are talking about here, there is a lot of force involved. I am really not an expert on stellar evolution, so I will have to pass you on to some referrences.

I am saying that there is a matter density beyond which the "matter is pulled together to the point where it ceases to be the element". This is what has happened in a neutron star. The gravitational force is so great that the protons and electrons of the atoms (of the star before the collapse) have combined into neutrons. (From particle physics, you get a neutron and a neutrino when a proton and electron interact with the weak nuclear force (exchange a W+)).

The trouble with finding the acceleration on the surface(event horizon) of a black hole, or a neutron star, is that the equations are from general relativity and not from the Newtonian physics that we are accustomed to. The gravity is so strong that time is changed in the vicinity of the object. Time is not an absolute. Currently, gravity is still a classical theory (in the sense that it has not been quantized). We know that quantum dynamics rules at the atomic level and smaller. Thus, I hate to speculate what happens to particles as the neutron star density increases beyond the critical point. This is where people like Hawking, and our own Werner Israel , come into the picture. They have the big brains to do the math involved. We are still waiting for someone like yourself to come along and figure everything out. I say go for it. (maybe read some of the papers out there to get a start)

Answer: I can safely say yes to one of your questions, there is an equation for determining the gravity on the surface of an object, it is F = G m_1 m_2 /(r^2) where F is the force between objects of mass m_1 and m_2 at a distance r apart. G is the gravitational constant to make the units work out. This is Newton's law of universal gravitation (remember the apple?). This does not take Einstein's theory of relativity into account.

Backing up a bit, a supernova is actualy the opposite of what you would expect. Rather than the "fusion exeeds the gravitational force", the nuclear fuel is exhausted and no longer able to support the star. The star colapses and creates a blast wave that ejects the star's envelope into space. This is not what happens at the end of a black hole's existence, but what happens at the end of a very large star's existence. After the explosion, a Supernova Remnant is all that remains. Most supernova reminants are thought to become neutron stars . Where are all the neutron stars? Only the largest of stars will end their lives in a nova or supernova. Smaller ones like out sun will just wither away.

Now, on to black holes. The theory is that if the supernova remnant is VERY massive, the gravitational forces overcome the pressure of the particles of the star. The star collapses to a point. That is, a singularity . I will let you look into black holes on your own, I am afraid to go near them. The radiation you speak of is a quantum effect . There is a considerable amount of information on black holes available on the web. At this point, I have only started to address your real point, which was the explosion at the end of the black hole's existence, but I am waiting until your project is done so I can put a link to it here . If you have the bandwidth, try out some Black Hole Movies.

Answer: There are several options available to you. The first would be to try asking some of your teachers at school . They may know of senior students who may be able to help you. There is a student help service here at UVic, but it is for first and second year university students (keep this in mind when you get to UVic). One place to try would be the UVic Physics and Astronomy Student Society ( PASS ). They maintain a list of Physics and Astronomy students that are willing to tutor other students. They are primarily there for other university students.

Answer: For sure in theory, support astronomy to find out in reality.

Answer: From the last two questions, I can see that technical admissions info is more on people's minds than pressing physics questions. Pertinent UVic links may be found on the Academics page. To get the final word, you should talk to some one in admissions (a disclaimer for me), but I have looked into this briefly. The university has set forth the following requirements for students.

Faculty of Science Admission Requirements; Students entering from British Columbia and Yukon Secondary Schools must satisfy the following minimum requirements:;..satisfactory completion of ...one of Francais 11, French 11, German 11, Japanese 11, Latin 11, Mandarin 11, Spanish 11; a beginner's language 11 course will not be accepted;

 

So it looks like the answer is yes, you do need the language 11 class. To get the final word, talk to someone in admissions. You MAY be able to take the language 11 course concurrently, in summer school, or from Camosun. Having the class waived is not likely, since the University has full enrollment. That is, even students meeting the minimum requirements are sometimes not accepted due to the large number of applicants. I urge you to contact admissions directly and explain your situation to them. They will be much more aware of any options that you might have. The UVic Admission Services number is listed as 721-8121 (8:30am until 4:30pm). Good Luck, I hope to see you next year.

Answer: It is fairly easy to apply at UVic. The registration forms are available here . You should be able to get these forms by conventional mail as well. EVEN BETTER... you can apply online right now (HINT: apply to science for a Physics degree).

The main UVic admission page has more detailed information. This is just the first step. After you get your admission package, you have to get ready to register in the classes you would like. It is often emphasized that it is your responsibility to register in the classes appropriate for your degree. Fortunately, there are many resources at your disposal. The calendar outlines what you should be taking for your chosen degree type. It is a good first resource, since it is semi-official. There is academic advising available. Finally, you can ask other students what the course structure is like.

Once you know what to take, you can register either by phone, or online via WebReg. Do NOT forget to visit the Scholarship Office and the Student Financial Aid Page.

Answer: The perception of observers traveling at speeds greater than about ten percent of the speed of light is always interesting, since many of the results are so unusual to our low speed view of the world. In this case, the observer would actually measure the refractive index of the tube to be the same as that of the stationary observer. This is due to length contraction and time dilation.

The measured index of refraction of the plastic, n, would be proportional to the velocity that the moving observer calculates for the light in the tube. Using d=v*t (or v=d/t in this case), the velocity calculated by the moving observer would be v=D/T=(d/gamma)/(t/gamma), where gamma is the velocity factor 1/sqrt(1-(v/c)^2), or 2.29 for v/c = 0.9 as in the question, d and t are the values for the observer in the rest frame of the tube and D and T are the values for the moving observer. The gamma factors cancel out, and the resulting velocity, which is proportional to n, would be the same.

Researchers have been able to slow light to speeds that do away with the need of a relativistic observer to travel faster than the light beam in the material. The current record is less than 17 m/s!

This sort of thing happens all the time to electrons. When a high speed electron enters a plastic or crystal, it's speed is reduced by electromagnetic interactions in the material. The resulting deceleration causes the radiation of energy in the form of light. Since the electron may still be moving at a speed close to the speed of light(in a vacuum), it may be traveling faster that the speed of light in the material. The result is very similar to the bow wave of a fast boat (since the boat is traveling faster than the propagation of the water waves). There is an intensified ring of light that propagates in a cone projecting into the direction of motion of the electron. This is known as Cherenkov radiation. Detection of Cherenkov rings is the method used by the Canadian SNO detector in the exploration of neutrinos. Here is a cool refraction applet.