Sunlight consists not only of light across the entire visible spectrum, but of invisible infrared and ultraviolet lights as well. The latter is probably what is causing the color-changing effects you mention.

Ultraviolet light is high-energy light, meaning that whenever it is emitted or absorbed, the amount of energy involved in the process is relatively large. Although light travels through space as waves, it is emitted and absorbed as particles known as photons. The energy in a photon of ultraviolet light is larger than in a photon of visible light and that leads to interesting effects.

First, some molecules can't tolerate the energy in an ultraviolet photon. When these molecules absorb such an energetic photon, their electrons rearrange so dramatically that the entire molecule changes its structure forever. Among the organic molecules that are most vulnerable to these ultraviolet-light-induced chemical rearrangements are the molecules that are responsible for colors. The same electronic structural characteristics that make these organic molecules colorful also make them fragile and susceptible to ultraviolet damage. As a result, they tend to bleach white in the sun.

Second, some molecules can tolerate high-energy photons by reemitting part of the photon's energy as new light. Such molecules absorb ultraviolet or other high-energy photons and use that energy to emit blue, green, or even red photons. The leftover energy is converted into thermal energy. These fluorescent molecules are the basis for the "neon" colors that are so popular on swimwear, in colored markers, and on poster boards. When you expose something dyed with fluorescent molecules to sunlight, the dye molecules absorbs the invisible ultraviolet light and then emit brilliant visible light.

Fluorescent paints and many laundry detergents contain fluorescent chemicals-chemicals that absorb ultraviolet light and use its energy to produce visible light. Fluorescent paints are designed to do exactly that, so they certainly contain enough "phosphor" for that purpose. Detergents have fluorescent dyes or "brighteners" added because it helps to make fabrics appear whiter. Aging fabric appears yellowish because it absorbs some blue light. To replace the missing blue light, the brighteners absorb invisible ultraviolet and use its energy to emit blue light.

Illumination matters because your skin only reflects light to which it's exposed. When you step into a room illuminated only by red light your skin appears red, not because it's truly red but because there is only red light to reflect.

Ordinary incandescent bulbs produce a thermal spectrum of light with a "color temperature" of about 2800° C. A thermal light spectrum is a broad, featureless mixture of colors that peaks at a particular wavelength that's determined only by the temperature of the object emitting it. Since the bulb's color temperature is much cooler than that of the sun's (5800° C), the bulb appears much redder than the sun and emits relatively little blue light. A fluorescent lamp, however, synthesizes its light spectrum from the emissions of various fluorescent phosphors. Its light spectrum is broad but structured and depends on the lamp's phosphor mixture. The four most important phosphor mixtures are cool white, deluxe cool white, warm white, and deluxe warm white. These mixtures all produce more blue than an incandescent bulb, but the warm white and particularly the deluxe warm white tone down the blue emission to give a richer, warmer glow at the expense of a little energy efficiency. Cool white fluorescents are closer to natural sunlight than either warm white fluorescents or incandescent bulbs.

To answer your question about shaves: without blue light in the illumination, it's not that easy to distinguish beard from skin. Since incandescent illumination is lacking in blue light, a shave looks good even when it isn't. But in bright fluorescent lighting, beard and skin appear sharply different and it's easy to see spots shaving has missed. As for makeup illumination, it's important to apply makeup in the light in which it will be worn. Blue-poor incandescent lighting downplays blue colors so it's easy to overapply them. When the lighting then shifts to blue-rich fluorescents, the blue makeup will look heavy handed. Some makeup mirrors provide both kinds of illumination so that these kinds of mistakes can be avoided.

You can produce colored flames by adding various metal salts to the burning materials. That's what's done in fireworks. These metal salts decompose when heated so that individual metal atoms are present in the hot flame. Thermal energy in the flame then excites those atoms so that their electrons shift among the allowed orbits or "orbitals" and this shifting can lead to the emission of particles of light or "photons". Since the orbitals themselves vary according to which chemical element is involved, the emitted photons have specific wavelengths and colors that are characteristic of that element.

To obtain a wide variety of colors, you'll need a wide variety of metal salts. Sodium salts, including common table salt, will give you yellow light--the same light that's produced by sodium vapor lamps. Potassium salts yield purple, copper and barium salts yield green, strontium salts yield red, and so on. The classic way to produce a colored flame is to dip a platinum wire into a metal salt solution and to hold the wire in the flame. Since platinum is expensive, you can do the same trick with a piece of steel wire. The only problem is that the steel wire will burn eventually.

Because the electrons in an atom move about as waves, they can follow only certain allowed orbits that we call orbitals. This limitation is equivalent to the case of a violin string--it can only vibrate at certain frequencies. If you try to make a violin string vibrate at the wrong frequency, it won't do it. That's because the string vibrates in a wave-like manner and only certain waves fit properly along the strong. Similarly, the electron in an atom "vibrates" in a wave-like manner and only certain waves fit properly around the nucleus.

Most of the collisions between an electron and a neon atom are completely elastic--the electron bounces perfectly from the neon atom and retains essentially all of its kinetic energy. But occasionally the electron induces a structural change in the neon atom and transfers some of its energy to the neon atom. In such a case, the electron rebounds weakly and retains only a fraction of its original kinetic energy. The missing energy is left in the neon atom, which usually releases that energy as light.

A mirror doesn't really flip your image horizontally or vertically. After all, the image of your head is still on top and the image of your left hand is still on the left. What the mirror does flip is which way your image is facing. For example, if you were facing north, then your image is facing south. This front-back reversal makes your image fundamentally different from you in the same way a left shoe is fundamentally different from a right shoe. No matter how you arrange those two shoes, they'll always be reversed in one direction. Similarly, no matter how you arrange yourself and your image, they'll always be reversed in one direction.

While you're looking at your image, the reversed direction is the forward-backward direction. But it's natural to imagine yourself in the place of your image. To do this you imagine turning around to face in the direction that your image is facing. When you turn in this manner, you mentally eliminate the forward-backward reversal but introduce a new reversal in its place: a left-right reversal. If you were to imagine standing on your head instead, you would still eliminate the forward-backward reversal but would now introduce an up-down reversal. Since it's hard to imagine standing on your head in order to face in the direction your image is facing, you tend to think only about turning around. It's this imagined turning around that leads you to say that your image is reversed horizontally.

The atoms in a molecule are usually held together by the sharing or exchange of some of their electrons. When two atoms share a pair of electrons, they form a covalent bond that lowers the overall energy of the atoms and sticks the atoms together. About half of this energy reduction comes from an increase in the negatively charged electron density between the atoms' positively charged nuclei and about half comes from a quantum mechanical effect--giving the two electrons more room to move gives them longer wavelengths and lowers their kinetic energies.

When two atoms exchange an electron, they form an ionic bond that again lowers the overall energy of the atoms and sticks them together. Although moving the electron from one atom to the other requires some energy, the two atomic ions that are formed by the transfer have opposite charges and attract one another strongly. The reduction in energy that accompanies their attraction can easily exceed the energy needed to transfer the electron so that the two atoms become permanently stuck to one another.

The red blood cells in your blood contain large amounts of a complicated and brightly colored molecule known as hemoglobin. This molecule's ability to bind and later release oxygen molecules is what allows blood to carry oxygen efficiently throughout your body.

Each hemoglobin molecule contains four heme groups, the iron-containing structures that actually form the reversible bond with oxygen molecules and that also give the hemoglobin its color. However, this color depends on the oxidization state of the heme group--red when the heme group is binding oxygen and blue-purple when the heme group is alone. That color difference explains why someone who is holding their breath may "turn blue"--their hemoglobin is lacking in oxygen. The clip you wore was analyzing the color of your blood to determine the extent of oxygenation in its hemoglobin. It measured your pulse rate by looking for periodic fluctuations in the opacity of your finger, brought on by changes in your finger's blood content with each heartbeat.

Both of your observations are correct: short wavelength light, such as violet, carries more energy per particle (per "photon") than long wavelength light, such as red, and red light does appear "warmer" than blue light. But the latter observation is one of feelings and psychology, rather than of physics. It is ironic that colors we associate with cold and low thermal energies are actually associated with higher energy light particles than are colors we associate with heat and high thermal energies.

First, an electromagnetic wave consists of an electric and a magnetic field. These two fields create one another as they change with time and they travel together through empty space. An electromagnetic wave of this sort carries energy with it because electric and magnetic fields both contain energy. That much was well understood by the end of the 19th century, but something new was discovered at the beginning of the 20th century: an electromagnetic wave cannot carry an arbitrary amount of energy. Instead, it can carry one or more units of energy, units that are commonly called "quanta." An electromagnetic wave that carries only one quanta of energy is called a "photon."

The amount of energy that a photon carries depends on the frequency of that photon--the higher the frequency, the more energy. Photons of visible light carry enough energy to induce various changes in atoms and molecules, which is why they provide our eyes with such useful information about the objects around us--we see how this visible light is interacting with the world around us.

Metal-halide lamps are actually high-pressure mercury lamps with small amounts of metal-halides added to improve the color balance. Light in such a lamp is created by an electric arc--electricity is passing through a gas in the lamp and causing violent collisions within the gas. These collisions transfer energy to the mercury and other gaseous atoms in the lamp and these atoms usually emit that energy as light. Overall, an electric current passes through the lamp and gives up most of its energy as light and heat in the gas. As you've noted, the lamp is relatively efficient, meaning that it produces more light and less heat than ordinary incandescent or halogen lamps. However, metal-halide lamps aren't quite as energy efficient as fluorescent lamps.

What makes a metal-halide lamp so efficient is that there are relatively few ways for the lamp to waste energy as heat. While collisionally excited mercury atoms normally emit most of their stored energy as ultraviolet light--the basis for fluorescent lamps--they can't do this in a high-pressure environment. A phenomenon called "radiation trapping" makes it almost impossible for this ultraviolet light to escape from a dense vapor of mercury, so a high-pressure mercury lamp emits mostly visible light. Even without the metal-halides, a high-pressure mercury lamp emits a brilliant blue-white glow. The metal-halides boost the reds and other colors in the lamp to make its light "warmer" and more like sunlight.

Next time you watch one of these lamps warm up, observe how its colors change. When it first starts up, its pressure is low and it emits mostly invisible ultraviolet light (which is absorbed by the lamp's glass envelope). But as the lamp heats up and its pressure increases, the rich, white light gradually develops. Incidentally, if the power to a hot lamp is interrupted, the lamp has to cool down before it can restart because it only starts well at low pressures.

While it is possible in principle to calculate the exact spectrum of light that a molecule will absorb, in practice it is normally extremely difficult. It's a matter of complexity--the quantum mechanical equations describing a molecule's electromagnetic structure are easy to write down but extraordinarily difficult to solve, even in approximation. One of the great challenges of atomic and molecular physics and physical chemistry is determining the full quantum mechanical structure of atoms and molecules through calculation alone. Except with small atoms and molecules, it's awfully hard but not impossible. As computers get faster and approximation schemes get better, the calculated spectra of molecules get closer to their experimental values.

As for compounds that change their optical properties while in electric fields, the answer is yes--all compounds exhibit such changes, although they may be undetectably small. However, I can't think of any isolated molecules that change dramatically in normal fields. Still, electric fields can alter the "selection rules"--the symmetry-based laws that often control which optical transitions can or cannot occur. It's possible that a modest electric field will turn on or off import optical transitions in some molecules so that they exhibit large color changes in small fields. Still, I can't think of any useful examples.

There is no doubt about it: light is both a particle and a wave. While it is traveling, light behaves as a wave--for example, it has a wavelength. But when it is being emitted or absorbed, light behaves as a particle--for example, it may transfer momentum, angular momentum, and energy to whatever it hits. A photon is a quantum of light, the smallest packet of light that can exist. You can't have half a photon of light--it's all or nothing. The amount of energy in a particular photon of light depends on the frequency (or wavelength) of that light.

Whenever you turn on a fluorescent lamp, a small amount of metal is sputtered away from the electrodes at each end of the tube. These electrodes are what provide electric power to the gas discharge inside the lamp and sputtering is a process in which fast moving ions (electrically charged atoms) crash into a surface and knock atoms out of that surface. Because sputtering is most severe during start up, a typical fluorescent tube can only start a few thousand times before its electrodes begin to fail. To avoid the expense and hassle of having to replace the tube frequently, you shouldn't cycle the lamp more than once every ten minutes. If you will only be away for a minute or two, leave the lamp on. But if you will be away for more than about ten minutes, turn it off. Incidentally, the claim that a fluorescent lamp uses a fantastic amount of electric power during start-up is nonsense. It's just a myth.

Light is both a particle and a ray (a wave). Its wave character was known and understood for many years before its particle character was discovered. That a film of clear soap exhibits colors is one of many demonstrations that light travels as waves, and such demonstrations were well understood in the 19th century. But it wasn't until the early 20th century that people discovered the particle character of light. They found that light is absorbed in discrete packets of energy or quanta, and these quanta of light energy were called photons. As a simple rule of thumb, you can think of light as exhibiting wave-like properties while it's traveling, but particle-like properties when it's being emitted or absorbed. This dual nature of light is complicated but unavoidable; it's a consequence of the quantum mechanical nature of our universe.

A neon light uses a very high voltage to propel an electric current through a low-density gas of neon atoms. These neon atoms are trapped inside a glass tube and the current passes between two metal electrodes at opposite ends of that tube. A high voltage power supply--typically a neon sign transformer--pumps a large number of negative charges onto one electrode and a large number of positive charges onto the other electrode. Because like charges repel while opposite charges attract, there are strong forces pushing the charges from one electrode toward those on the other electrode. Eventually, charges at the two ends of the tube begin to leap off the electrodes and into the neon gas so that they can flow toward one another. Current begins to flow through the tube. As the charges move through the gas, they frequently collide with neon atoms and occasionally transfer some of their energies to those neon atoms. During such an energy transfer, an electron in the neon atom shifts from its normal orbital to a higher energy orbital in which the electron doesn't normally travel. The electron soon returns to its normal orbital and releases a particle of light--a photon--in the process. Since the most common orbital shift in an excited neon atom releases a particle of red light, a neon light emits a bright, reddish glow.

The terms "resonant" and "resonate" are general expressions that refer to repetitive motions or actions that occur spontaneously within a system. Elements exhibit many different resonant behaviors in different situations, so I must pick an appropriate resonant behavior in order to answer your question.

The best choice I can think of is nuclear magnetic resonance (NMR)--an effect that involves the flipping of an atomic nucleus's magnetic poles. Most atomic nuclei--the massive positively charged nuggets at the centers of atoms--are magnetic. When you put an atom with a magnetic nucleus in a magnetic field, the atom acquires a certain amount of potential energy that depends on whether that magnetic nucleus is aligned with the magnetic field or not. The extent to which the atom's nucleus is aligned with the field can be changed by exposing it to an electromagnetic wave of the right frequency. This electromagnetic wave provides or absorbs the required energy to allow the nucleus's magnetization to flip. The nucleus exhibits a resonance in response to the correct electromagnetic wave--a phenomenon called "nuclear magnetic resonance." This frequency at which this resonance occurs depends on the nucleus, on the magnetic field, and on the magnetic environment of the nucleus. The resonance occurs for any magnetic nucleus, in any field, but how interesting or useful the resonance is depends on the situation. So the answers to both questions are yes, but that doesn't mean the effects are important.

My answer to that question depends on the level of detail you're interested in. As an example of what I mean by that statement, imagine describing what a simple house is made of. At the coarsest level, you might say that it consists of a floor, a ceiling, four walls, and a roof. At a greater level of detail, you might say that it consists of many boards, some tarpaper, and lots of nails. At a still finer level of detail, you might say that it consists of atoms and molecules, and... you get the point. So it is with atoms. I'll answer the question at a fairly coarse level of detail, one that's familiar to many people, and then say a word or two about the next level of detail.

The principal constituents of an atom are protons, neutrons, and electrons. These are three most important subatomic particles; the main building blocks of matter in the same way that wood, bricks, and steel are the major building blocks of houses. Each of these particles has a mass--the measure of their inertia--and two of them, electrons and protons, are electrically charged. Each electron has one unit of negative charge while each proton has one unit of positive charge. Because an atom is normally electrically neutral--its positive and negative charges must balance--it has an equal number of electrons and protons. The number of neutrons in an atom is somewhat flexible.

These particles, electrons, protons, and neutrons, are held together by several types of forces. The protons and neutrons, which are relatively massive, stick to one another at the center of the atom and form a dense object called the atomic nucleus. The particles in the nucleus are held together by the "nuclear" force, which binds together protons and neutrons that are touching one another. This nuclear force is quite strong and is able to overcome the strongly repulsive electromagnetic forces that the protons in the nucleus exert on one another--like electric charges repel one another and the protons are all positively charged. The electrons circulate around the atom's nucleus, held in place by the strongly attractive electromagnetic forces that protons exert on electrons--opposite electric charges attract one another and the electrons are negatively charged while the protons are positively charged.

The electrons do most of the circulating around the nucleus, rather than the other way around, because they are much less massive than the nucleus. As with the planets around the sun, the less massive objects tend to orbit the more massive objects. At a basic level, you can view an atom as a tiny solar system with its neutrons and protons at the center and its electrons orbiting around this central nucleus. Quantum physics dramatically complicates this picture, but it's a helpful picture nonetheless.

At the next level of detail, the protons and neutrons themselves have structure--they are built out of yet smaller particles known as quarks. The particles also stick to one another by tossing particles back and forth--particles including photons and gluons. But that is a whole new story.

While incandescent lighting isn't nearly as energy efficient as those other light systems, it produces a more eye pleasing light than some of the alternatives. Our eyes are optimized for sunlight, so that we find the spectrum of light from hot objects particularly pleasant. The heart of an incandescent bulb is a hot tungsten filament. High-pressure arc lamps such as sodium vapor or mercury vapor lamps (metal halide lamps are just somewhat color-corrected high pressure mercury vapor lamps) produce a much less even spectrum of light. High-pressure sodium vapor lamps are wonderfully energy efficient, but their light is orange or pink. High-pressure mercury vapor lamps are also quite energy efficient, but their light is somewhat bluish. Even metal halide lamps aren't quite white. The other problem with high-pressure arc lamps is that they take time to warm up and then can't be restarted until they cool off. They're best in applications that don't require them to be turned on or off frequently.

A much better choice, both in terms of energy efficiency and light color, is a fluorescent or compact fluorescent lamp. Such lamps typically use less than 25% of the energy required for comparable incandescent lighting, provide excellent color rendering that can be chosen to match that of incandescent lighting, and they last much longer than incandescent bulbs. Even though compact fluorescent lamps are more expensive than incandescent bulbs up front, they last so much longer and save so much energy that each one typically saves you about $45 over its working life.

A neon light uses a high voltage transformer to place electric charges on the wires at each end of a neon-filled glass tube. One end of the tube receives positive charges and the other end receives negative charges. Since like charges repel one another, the vast numbers of like charges at each end push apart strongly and some of them leave the wire and enter the neon gas. Once they're in the gas, these charges are draw quickly toward the opposite charge at the far end of the tube. As they travel through the tube, these moving charges pick up speed and kinetic energy but they occasionally collide with neon atoms as they travel and can transfer some of their kinetic energies to the neon atoms. The neon atoms retain this extra energy only briefly before getting rid of it in the form of visible light--the familiar red glow of a neon lamp. Overall, electric charges stream from one end of the tube to the other, frequently colliding with the neon atoms and causing those atoms to emit red light. If you look closely at a neon lamp, you'll see that it is the gas itself that's emitting the red light.

The fact that all objects, including people, travel as waves in our universe is probably not what the writers of Startrek had in mind when they "invented" the transporter. In Startrek, the transporter seems to disassemble the people involved at one location and then reconstruct them at another. That disassembly/reassembly process is purely science fiction while the wave propagation of matter is quite real. We never notice this wave propagation for large objects because their wave effects are too small to detect and because watching an object propagate prevents its wave properties from having any significant consequences. Each observation of an object tends to localize it and minimize its wave properties, so that watching an object moves makes the effects of its wave properties minimal.

An atom in a gas discharge emits light when one of its electrons shifts from an orbital with extra energy into an empty orbital in which it will have less energy. Since an electron can only travel around the atom's nucleus in an allowed orbit--an orbital--and the energy it has while in that orbital is very specifically defined, such a shift from one orbital to another results in the emission of a photon of light with a very specific energy. Because a photon's energy is directly proportional to the frequency of the light, and light's frequency and wavelength are related by the speed of light, the amount of energy the electron gives up in shifting from one orbital to another determines the photon's energy, frequency, and wavelength.

A strobe light passes a brief, intense pulse of electric current through a gas, which then emits a brilliant burst of light. The gas is usually one of two inert gases, xenon or krypton, that emit relatively white light when they're struck by the fast moving electrons in the electric current. When it hits a xenon or krypton atom, an electron may give up some of its kinetic energy--its energy of motion--to the electrons in the atom. Those atomic electrons shift from their usual orbitals (quantum mechanically allowed orbits) to higher-energy orbitals that they usually don't travel in. The atomic electrons remain only briefly in these higher-energy orbitals before dropping back to their original orbitals. As they drop back down, these electrons give up their extra energy as light. Because krypton and xenon atoms have a great many electrons and their electronic structures are very complicated, they emit light over a broad range of wavelengths. Moreover, the gases are at relatively high pressures and collisions between the atoms while they are emitting light further smooth out the spectrum of light they produce. Thus the strobe emits a rich, white light during the moments while current is passing through the gas.

Supplying the enormous current needed to maintain the brief arc in the strobe's gas is done with the help of a capacitor, a device that stores separated electric charge. A high voltage power supply pumps positive charge from the capacitor's negative plate to its positive plate, until there is a huge charge imbalance between those two plates. You can often hear a whistling sound as this power supply does its work. The capacitor plates are connected to one another through the gas-filled flashlamp that will eventually produce the light. However, current can't pass through the gas in the flashlamp until some electric charges are injected into the gas. These initial charges are usually produced by a high voltage pulse applied to a wire that wraps around the middle of the flashlamp. When a few charges are inserted into the gas, they accelerate rapidly toward the positive or negative wires that extend from the charged capacitor. As these charges pick up speed, they begin to collide with the gas atoms and they deposit energy in those atoms. Electrons are occasionally knocked out of atoms or out of the wires at the end of the flashlamp and these new charges that enter the gas also begin to accelerate toward the wires. A cascade of collisions quickly leads to a violent arc of charged particles flowing through the flashlamp and colliding with the gas atoms. The flashlamp emits its brilliant burst of light that terminates only when the capacitor's separated electric charges and stored energy are exhausted.

All objects in our universe have wave-like characteristics that manifest themselves in certain circumstances. These wave-like characteristics become more significant as objects become smaller. Their wave-like characteristics allow small particles to have ill-defined locations. To understand what I mean by "ill-defined locations", consider a wave on the surface of a lake. There is no one point at which this wave is located--it is located over a region of the water's surface. Waves don't have well defined locations. Similarly, if you observe an electron, which is really a wave, there is no one point at which that electron is located--it is located over a region of space. Because of the detailed relationships between wavelength, frequency, and energy, the smaller the region of space in which the electron-wave can be found, the higher its energy must be. Thus an electron that is localized at all--that is known to be within a certain region of space--must have a certain minimum energy, even if it is stationary. This minimum energy is called zero point energy and it is a consequence of trying to localize the particle within a certain region of space. Since the zero point energy is a base level and can't be reduced, you can't use zero point energy to do anything useful. It's just there.

I think that most black lights are gas discharge lamps that resemble normal fluorescent lamps. However, while a normal fluorescent light uses fluorescent phosphors to convert the ultraviolet light produced by its mercury discharge into visible light, a black light allows that ultraviolet light to emerge from the lamp unchanged. The ultraviolet light from a mercury discharge has too short a wavelength to be useful or safe as artistic black light, so other gases are likely to be used. The lamps are probably filtered so that they emit relatively little visible light or short wavelength ultraviolet light.

The spectrum of light from an incandescent bulb is what is known as a blackbody thermal spectrum--the light produced by a hot object. A blackbody spectrum is relatively featureless--you can't even tell what material is producing the light; only what temperature it has. All the wavelengths of light are present in thermal radiation and their intensities vary smoothly with wavelength. For the filament temperature of a normal incandescent bulb, the reds are brighter than the greens and the blues are rather weak.

A fluorescent bulb pieces together white light out of several separate colored lights. The spectrum of light from a fluorescent lamp is not simple or featureless--many wavelengths are essentially missing and the intensities of the remaining wavelengths don't vary smoothly with wavelength. Viewed through a spectroscope, the light from a fluorescent light has many bright bands of color interspersed with relatively dark bands.

Food coloring is a solution of dye molecules--molecules that absorb light of certain wavelengths extremely efficiently. When a particle of light--a photon--of the right wavelength encounters one of these dye molecules, an electron in the molecule uses the photon's energy to shift from one quantum level to another. The photon vanishes and the molecule is placed in an electronically excited state. The dye molecule's electron quickly returns to its original quantum level by releasing this extra energy as thermal energy within the molecule and its surroundings. Overall, the photon has vanished and the dye has become warmer. When you add these dye molecules to food, the dye gives the food a color by preventing that food from transmitting or reflecting certain colors of light. The dye simply absorbs those colors.

An electric field can always been shielded by encasing its source in a grounded conducting shell. Electrically charged particles in the shell will naturally rearrange themselves in such a way as to cancel the electric fields outside the shell. But magnetic fields are harder to shield, particularly if they don't change very rapidly with time. The difficulty with shielding magnetic fields comes from the apparent absence of isolated magnetic poles in our universe--there is no equivalent of electrically charged particles in the case of magnetism. As a result, the only way to shield magnetic fields is to take advantage of the connections between electric and magnetic fields.

Because changing magnetic fields are always accompanied by electric fields, the two can be reflected as a pair by highly conducting surfaces or absorbed by poorly conducting surfaces. In these cases, the electric fields push and pull on electric charges in the surfaces and it is through these electric fields that the magnetic fields are reflected or absorbed. However, this effect works much better at high frequencies than at low frequencies, where very thick materials are required. Appliances that operate from the AC power line have magnetic fields that change rather slowly with time (only 120 reversals per second or 60 full cycles of reversal each second) and that are extremely hard to shield with conducting material. Instead, their magnetic fields have to be trapped in special magnetic materials that draw in magnetic flux lines and keep them from emerging into the surrounding space. One of the most effective magnetic shield materials is called "mu metal", a nickel alloy that's like a sponge for magnetic flux lines. Since it also conducts electricity pretty well, it is an effective shield for electric fields. So if you wrap your mercury vapor ballasts in mu metal, there would be almost no electric or magnetic fields detectable outside of the mu metal surface.

The true primary colors of light are Red, Green, and Blue. This empirical result is determined by physiological characteristics of the three types of color sensitive cells in our eyes. These cells are known as cone cells and are most sensitive to red light, green light, and blue light respectively. Light that falls in between those wavelength ranges stimulate the three groups of cells to various extents and our brains use their relative stimulations to assign a color to the light we're seeing. For example, when you look at yellow light, the red sensitive and green sensitive cone cells are stimulated about equally and your brain interprets this result as yellow. When you look at an equal mixture of red light and green light, the red sensitive and green sensitive cells are again stimulated about equally and your brain again interprets this result as yellow. Thus you can't tell the difference between true yellow light and an equal mixture of red light and green light. That's how a television tricks your eyes into seeing all colors. If you look closely at a color television screen, you'll see tiny dots of red, green, and blue light. But when you back up, you begin to see a broad range of colors. The television is mixing the three primary colors of light to make you see all the other colors.

Incidentally, the three primary colors of pigment are yellow, cyan, and magenta. Yellow pigment absorbs blue light, cyan pigment absorbs red light, and magenta pigment absorbs green light. When exposed to white light, a mixture of these three pigments controls the mixture of the reflected lights (red, green, and blue) and thus can make you see any possible color.

When electric current passes through air as an arc, the air becomes hot enough to vaporize the compounds you expose to it. As a result, there are individual sodium and chlorine atoms moving about in the arc itself. Like all atoms, a sodium atom resembles a tiny planetary system. It has 11 negatively charged electrons orbiting a massive, positively charged nucleus. But unlike our experience with the solar system, the electrons in a sodium atom can only travel in certain allowed orbits or "orbitals." These electrons are normally found in the orbitals with the lowest possible energy. But when charged particles in the arc collide with sodium atoms, they often shift electrons in those atoms to orbitals with more energy. The electrons quickly return to their original orbits and emit their excess energies as light during their returns. In the case of sodium, the final step of the most common return path results in the emission of yellow light with a wavelength of about 590 nanometers. This yellow light is the same one you see in the sodium vapor lamps that are used to light highways and parking lots.

While sodium tends to emit yellow light, other atoms have different orbital structures and emit their own characteristic colors. Copper and barium atoms emit blue/green light while strontium atoms emit red light. These colored lights are the same ones that you see in fireworks.

Luminol produces light during a chemical reaction with either molecular oxygen or a mixture of potassium ferricyanide and hydrogen peroxide and is probably the basis for most light sticks. In an alkaline (basic) solution, the luminol molecule becomes a dianion, a molecule with two negative charges on it. In this dianion form, the molecule has two nitrogen atoms exposed to the solution and these nitrogen atoms are easily replaced by two oxygen atoms. When that exchange takes place, a molecule of nitrogen gas is released and the final oxidized luminol is left in an electronically excited state. This molecule quickly gets rid of its excess energy by emitting light.

Most glow-in-the-dark materials store energy when they are exposed to visible light and then glow dimly as this stored energy is gradually converted back into light. In such a material, exposure to light promotes some of the electrons in the atoms or molecules to excited states and these electrons become trapped in lower-energy excited states from which they have trouble escaping. It takes a very long time for each of these trapped electrons to return to their original states by emitting light. Since that return is a random process, a glow-in-the-dark object glows with an ever diminishing light as the excited electrons return at random moments to their original states. Eventually almost all the electrons have returned and the glow weakens to essentially nothing.

White phosphorus also glows in the dark, but not for the same reason. You don't need to expose white phosphorus to light to make it glow; you need to expose it to air. The chemical reaction between phosphorus and oxygen causes the phosphorus to emit light. This reaction can also cause the white phosphorus to burst into flames. Because of its dangerous flammability and its toxicity, white phosphorus isn't something you want to have around.

A neon lamp consists of a neon-filled tube with an electrode (a metal wire) at each end. When you put enough electrons on one of the electrodes and remove enough electrons from the other, electrons will begin to leap off the first electrode and accelerate toward the other electrode. Because the density of neon atoms in the tube is relatively low, only about 1/1000th that of air molecules in normal air, the electrons can travel long distances without colliding with a neon atom. As the electrons accelerate, their kinetic energies increase. However, these electrons occasionally collide with neon atoms and, when they do, they can give up some of their kinetic energies to those atoms. The neon atoms then end up with excess energy and they often emit this energy as light. The color of this light is determined by the structure of a neon atom and tends to be the familiar red of a neon sign.

I believe that the glow sticks contain luminol and hydrogen peroxide, which mix when you crack the glass ampoule and begin to emit light. There are several other chemicals present in the sticks to assist and control the process, but the principal reaction is one in which the hydrogen peroxide oxidizes ("burns") the luminol molecule. The result is a product molecule that is initially in an excited state--its electrons have more energy than they need--and it emits a particle of bluish-violet light. Since our eyes aren't particularly sensitive to that bluish-violet light, it's often converted into more visible light with the help of a fluorescent dye. The green light sticks probably contain sodium fluorescein molecules, each of which can absorb a photon of bluish-violet light and reemit some of its energy as a photon of green light. Other dyes, probably rhodamines, are used to make red or orange light sticks.

I would guess that the lower wattage tubes will work fine in your existing fixtures, but I am not expert enough to be certain. The 25W tube itself is evidently built so that a smaller current flows through it than through a normal 40W tubes when the two are exposed to similar voltages. The ballast's job is to prevent a catastrophic rise in that current by adjusting the voltage across the tube dynamically during each half cycle of the power line and to keep the tube operating even as a half cycle is coming to an end. Although the 25W tube will draw less current than the ballast expects, the ballast should behave pretty well. I would expect that the tube designers have anticipated this situation and have built the tube to operate with the standard ballast. If a reader knows better, please let me know.

Not unless you will consider a black hole to be a black bulb. For a "bulb" to absorb light that isn't heading toward the bulb, that bulb will have to attract the light toward it. Since light has no electric charge, the only force that the bulb can exert on light is gravitational force. While a black hole's gravity is strong enough to attract and ensnare light, it wouldn't make a very practical bulb. However, it is possible in certain circumstances to add light to previously existing light and, in doing so, create a dark shadow that wasn't present before. This process is called interference, where two light waves cancel one another in a particular region of space and prevent any light from reaching a certain spot. But this cancellation is difficult to achieve, except with lasers, and doesn't occur everywhere in space--the light doesn't vanish, it just gets redistributed. Overall, the idea of a black bulb is just not realistic.

There are a few molecules that can be chemically oxidized to produce new molecules that then spontaneously emit light. The chemical reactions that occur in these special molecules leave the resulting new molecules electronically excited--their electrons are in states that have more than the minimum allowed energies. As these energetic electrons subsequently shift to states with less energy, they release some of that energy as light.

In a firefly, the molecule that is being oxidized is called luciferin. It's combined with oxygen and the important biological energy storage molecule ATP (adenosine triphosphate), assisted by a catalyst protein called luciferase. A series of reactions then occurs, culminating in the formation of excited decarboxyketoluciferin. This molecule emits a photon of green light and becomes normal decarboxyketoluciferin.

Luminol, a molecule used in many cold light products, is a somewhat simpler molecule that is much easier to synthesize commercially than is luciferin. When it's oxidized with hydrogen peroxide and potassium ferrocyanide, it forms an excited molecule that emits a photon of blue light. This blue light is often shifted to green or orange with the help of a fluorescent dye. The dye absorbs the blue light and uses its energy to emit green or orange light. This material is commonly used in light sticks and glowing necklaces or toys.

Since plants appear green, they are absorbing mostly the red and blue portions of the visible light spectrum. Blue light is particularly important to them. Incandescent light contains relatively little blue light, so it probably doesn't help plants very much. Because fluorescent lighting provides more blue light than incandescent lighting, fluorescent lighting is certainly better for plants.

Light is an electromagnetic wave--an excitation of the electric and magnetic fields that can exist even in "empty" space. Light's electric field creates its magnetic field and its magnetic field creates its electric field and this self-perpetuating arrangement zips off through space at a phenomenal speed--the speed of light. Light is created by moving electric charges, which first excite the electromagnetic fields. Light is also absorbed by electric charges, which obtain energy from the light's electromagnetic fields.

Like everything else in the universe, light exhibits both wave and particle behaviors. When it is traveling through space, light behaves as a wave. That means that its location is generally not well defined and that it can simultaneously pass through more than one opening (the way a water wave can when it encounters a piece of screening). But when light is emitted or absorbed, it behaves as a particle. It's created all at once when it's emitted from a particular location and it disappears all at once when it's absorbed somewhere else. This wave/particle arrangement is true of everything, including objects such as electrons or atoms: while they are traveling unobserved, they behave as waves but when you go looking for them, they behave as particles.

The starting process erodes the electrodes of a fluorescent tube through a phenomenon called sputtering. A typical fluorescent tube will last about 50,000 hours if left on continuously but only 20,000 hours if it's turn on for just 3 hours at a time. From that tidbit, I think its fair to say that a fluorescent tube can only start about 10,000 times. If the tube costs $5, you are spending about 0.005 cents per start. If the electricity to operate that tube costs about 0.2 cents per hour, then turning the tube off for about 1.5 minutes saves the same amount of money in electricity as it costs in tube life when you turn the tube back on. In short, if you turn the lamp off for less than about 1 minute, you're wasting money. But if you turn it off for more than 10 minutes, you're saving money. In between, it's not so clear. There is a myth that turning on a fluorescent lamp consumes a huge amount of electricity so that you shouldn't turn the lamp off and on. There is simply no basis to that myth.

The exact composition depends on the color type of the bulb, with the most common color types being cool white, warm white, deluxe cool white, and deluxe warm white. In each case, the phosphors are a mixture of crystals that may include: calcium halophosphate, calcium silicate, strontium magnesium phosphate, calcium strontium phosphate, and magnesium fluorogermanate. These crystals contain impurities that allow them to fluoresce visible light. These impurities include: antimony, manganese, tin, and lead.

While the electric discharge in the tube's mercury vapor emits large amounts of short wavelength ultraviolet light, virtually all of this ultraviolet light is absorbed by the tube's internal phosphor coating and glass envelope. As a result, a fluorescent lamp emits relatively little ultraviolet light. I think that the ultraviolet light level under fluorescent lighting is far less than that of outdoor sunlight.

While the phosphors in fluorescent lamps are not considered to be toxic, they do contain a tiny amount of mercury. This mercury is an essential part of the operation of the lamp (it is what creates the initial light during the electric discharge). While most fluorescent lamps are simply discarded into landfill, some facilities (including the University of Virginia) dispose of them more carefully. The University of Virginia breaks the lamps to collect the phosphors and then distills the mercury out of the phosphors. The phosphors are then entirely non-hazardous and the mercury is recycled.

When you bend a plastic light stick, you break a small glass ampoule and allow two chemicals that are contained inside the stick to mix. One of these chemicals is a powerful oxidizing agent and the other is a chemical that when oxidized ("burned") is left in an electronically excited state. In other words, the chemical reaction between the molecules of the two chemicals creates a new molecule that has excess energy in it. The molecule releases this energy as a particle of light, a photon. Although I am not certain exactly which chemicals are used in a modern light stick, I believe that one is hydrogen peroxide (the oxidizer) and the other is luminol (the chemical that is oxidized). Upon oxidization, luminol emits a photon of blue or ultraviolet light. The green light that you see emerging from a typical light stick is actually a second photon that is emitted by a fluorescent dye contained in the light stick. This dye absorbs the blue or ultraviolet photon emitted by the luminol and then reemits a new photon with somewhat less energy and a green color.

Fluorescence is the prompt emission of light from an atom, molecule, or solid that has extra energy. For example, when some of the dyes used in modern swimwear and clothing are exposed to ultraviolet light, they absorb the light energy and promptly reemit part of that energy as visible light--typically brilliant greens and oranges. In contrast, phosphorescence is the delayed emission of light by an atom, molecule, or solid that has extra energy. Glow-in-the-dark objects are phosphorescent--they are able to store the extra energy they obtain during exposure to light for remarkably long times before they finally release that stored energy as visible light. Systems that exhibit phosphorescence rather than fluorescent are those that have special high-energy states that have enormous difficulty radiating away energy as light. Finally, triboluminescence is the emission of light from a surface experiencing sliding friction. Since sliding friction introduces energy into the surfaces that are sliding across one another, it's possible for that energy to be emitted as light.

Fluorescent lights work by sending an electric current through a vapor of mercury atoms in what is known as an electric discharge. Unfortunately, electric discharges are very unstable--they are hard to start and, once started, tend to draw more and more current until they overheat and damage their containers and power sources. Thus a fluorescent light needs some device to control the flow of current through its discharge. Since normal fluorescent lamps are powered by alternating current--that is, the current passing through the discharge stops briefly and then reverses direction 120 times each second in the United States and 100 times each second in many other countries (60 or 50 full cycles of reversal, over and back, each second respectively)--the current control device only needs to keep the current under control for about 1/120 of a second. After that the current will reverse and everything will start over.

Older style fluorescent lights use a magnetic ballast to control the current. This ballast consists essentially of a coil of wire around a core of iron. As current flows through the wire, it magnetizes the iron. Because energy is required to magnetize the iron, the presence of the iron inside the coil of wire slows down the current when it first appears in the wire by drawing energy out of that current. This effect, typical of devices known to scientists and engineers as "inductors", prevents the current passing through the ballast and then through the discharge from increasing too rapidly once it starts. The magnetic ballast is able to slow the current rise through the fluorescent lamp long enough for the alternating current to begin reversing directions. In fact, as the current in the power line begins to reverse, the ballast begins to get rid of the energy stored in its magnetized core. This energy is used to keep the discharge going longer than it would on its own. The ballast thus smoothes out the discharge so that it stays under control and emits an almost steady amount of light.

Modern electronic ballasts still control the current through the discharge, but they use electronic components to achieve this control. Just as an electronic dimmer switch can control the current through an incandescent light bulb in order to adjust the bulb's brightness, such electronic devices can control the current passing through the discharge in a fluorescent lamp to keep that current from growing dangerously large.

While a converging lens or a concave mirror can always direct light from a bright source in a particular direction, the degree of collimation (the extent to which the rays become parallel) depends on how large the light source is. The smaller the light source, the better the collimation. Spotlights and movie projects use extremely bright, very small light sources to create their highly collimated beams. Since fluorescent lamps tend to be rather large and have modest surface brightnesses, I'm afraid that you would be disappointed with the best beam that you could create from that light. The ultimate collimated light source is a laser beam. In effect, the identical photons of light in a laser beam all originate from the same point in space, so that the collimated beam is as close to perfectly collimated as the nature of light waves will allow.

At low pressure, a mercury vapor lamp emits mostly short wavelength ultraviolet light at a wavelength of 254 nanometers. This light comes from the dominant atomic transition in the mercury atom, between its first excited state and its ground state. However, as the pressure and density of mercury atoms inside the lamp increase, two things happen. First, the high density of mercury atoms in the lamp makes it difficult for the 254-nanometer light to escape from the lamp. Each time a 254-nanometer photon (particle of light) is emitted by one mercury atom, a nearby mercury atom absorbs it. As a result, the 254-nanometer light becomes trapped inside the lamp and diminishes in brightness. With so much energy trapped inside the lamp, the mercury atoms are able to reach more highly excited states than at low density. Second, frequent collisions between the now highly excited mercury atoms allow those mercury atoms to emit wavelengths of light that are normally forbidden in the absence of collisions. The mercury atoms begin to emit light at a wide variety of wavelengths, including substantial amounts of visible light. That's why a high-pressure mercury lamp is a brilliant source of visible light--most of the ultraviolet light is trapped by the mercury vapor and a substantial fraction of the light emerging from the lamp is visible light.

I'm not aware of any tendency to change colors as it begins to burn out, but many fluorescent bulbs are relatively blue in color. The phosphor coatings used to convert the mercury vapor's ultraviolet emission into visible light don't create pure white. Instead, they create a mixture of different colors that is a close approximation to white light. There are a number of different phosphor mixtures, each with its own characteristic spectrum of light: cool white, deluxe cool white, warm white, deluxe warm white, and others. The cool white bulbs are most energy efficient but emit relatively bluish light. This light gives the bulbs a cold, medicinal look. The warm white bulbs are less energy efficient, but more pleasant to the eye.

At low pressure, a mercury lamp emits mostly 254-nanometer ultraviolet light. That light is created when an electron in the mercury atom goes from its lowest excited orbital to its ground (normal) orbital. The other wavelengths of light emitted by the low-pressure lamp are weak and widely spaced in wavelength. An electron must sent into a very highly excited orbital in order to emit one of these other wavelengths. But at high pressure, mercury atoms have trouble sending their favorite 254 nanometer light out of the lamp. Whenever one of the atoms emits a particle of 254-nanometer light (moving its electron from the first excited orbital to the ground orbital), another nearby atom absorbs that particle of light (moving its electron from the ground orbital to the first excited orbital). As a result the 254-nanometer light cannot escape from the lamp; it becomes trapped in the mercury gas! Instead, the atoms begin to send their energy out of the lamp by concentrating on radiative transitions between highly excited orbitals and that lowest excited orbital. These wavelengths become more common in the light emission from the lamp as its pressure rises. But some radiative transitions that are forbidden at low pressure (that cannot occur because an electron is not able to move from one particular excited orbital to another particular excited orbital) become allowed at high pressure. Collisions break many of the rules that govern atomic behavior, allowing otherwise forbidden events to occur. In the case of the mercury lamp, collisions at high pressure permit the mercury atoms to emit wavelengths of light that they cannot emit a low pressure when collisions are rare.

The lamp first heats the filaments in its electrodes red hot so that they begin to emit electrons and then tries to start a discharge across the lamp. If there are not enough electrons leaving the electrodes to sustain a steady discharge, the lamp will blink briefly but will not stay on. The lamp will try again; first heating its filaments and then trying to start the discharge. The lamp may blink several times before the discharge becomes strong enough to keep the electrodes hot and sustain the discharge.

Fluorescent tubes operate at very low pressure; roughly 1/1000th of an atmosphere. They do not explode when broken; they implode. The atmospheric pressure surrounding the tube crushes it as soon as it begins to crack. The tube shape of a typical fluorescent tube is chosen because it can withstand the enormous compressive forces of the atmosphere better than most other shapes.

When an atom is excited by a collision and then emits energy as light, it converts most of the collision energy into light. Thus the gas in a fluorescent lamp experiences many collisions but emits most of the collision energy as light. The gas becomes slightly hot, but not nearly as hot as the filament of an incandescent bulb. The electrical energy arrives at the fluorescent bulb as a current of charged particles and most of this energy leaves the bulb as light, without ever becoming heat. However the electrical energy arriving at an incandescent bulb becomes heat first and then becomes light. The conversion of electrical energy to heat dramatically reduces the bulb's ability to emit visible light efficiently.

Yes. The extra energy is converted into heat by the phosphors. Their electrons absorb the light energy, convert some of that energy into heat, and then reemit the light. Since the new light contains less energy per particle (per photon) than the old light, it appears as visible rather than ultraviolet light.

Fluorescent lamps do not operate well in extreme cold. Below about 15° C (59° F), the density of mercury atoms in the tube's vapor is too low to produce efficient light. While the tube also contains inert gases that allow it to start at almost any temperature, the scarcity of mercury atoms leads to a reduced light output. In any case, the electrodes must be heated to make them emit electrons to sustain the discharge.

The optimal internal temperature for a fluorescent lamp is about 60° C (140° F). The tube reaches this internal temperature when its outside is about 40° C (104° F). When the surrounding temperature exceeds 40° C, the tube begins to waste energy again because the density of mercury atoms in the vapor becomes too large.

While there is mercury in a fluorescent lamp, the amount of mercury is relatively small. There are only about 0.5 milligrams of mercury in each kilogram of lamp, or 0.5 parts per million. In fact, because fluorescent lamps use so much less energy than incandescent lamps, they actually reduce the amount of mercury introduced into our environment. That's because fossil fuels contain mercury and burning fossil fuels to obtain energy releases substantial amounts of mercury into the environment. If you replace your incandescent lamps with fluorescent lamps, the power company will burn less fuel and release less mercury. That's one reason to switch to fluorescent lamps, even if you must simply throw those lamps away when they burn out. Nonetheless, there are programs to recycle the mercury in fluorescent lamps. Last year, the University of Virginia recycled 31 miles of fluorescent lamps. They distilled the mercury out of the white phosphor powder on the inner walls of the tubes. Once the mercury has been removed from that powder, the powder is not hazardous. The university also recycled the glass. One last note: the mercury is an essential component of the fluorescent lamp--mercury atoms inside the tube are what create ultraviolet light that is then converted to visible light by the white phosphor powder that covers the inside of the tube.

Sustaining the discharge in a gas lamp requires the steady production of charged particles. Even if a lamp contains many negatively charged electrons and positively charged ions, these particles will quickly migrate to the electrodes once electric fields are present in the tube. If they don't produce more charged particles as they fly across the tube, these charged particles will quickly disappear and the discharge will stop. It takes a critical number of charged particles in the tube to ensure a steady production of new charged particles. Thus the tube may not always start, even if it has a brief flicker of light.

The electrodes age, particularly during start up. The endless bombardment of charged particles gradually chips or "sputters" material off of the electrodes until they are no longer able to sustain a steady discharge. The final blow usually occurs when the heater filament breaks and the lamp cannot be started at all.

A fluorescent lamp produces light as the result of an electric discharge that takes place inside the lamp tube. Electrons, emitted from hot filaments at each end of the tube, are pulled through the tube by electric fields and collide violently with mercury atoms inside the tube. These mercury atoms then emit ultraviolet light, which is converted to the visible light you see by the phosphor coating inside the glass tube.

To emit the electrons needed to sustain the discharge, the filaments at each end of the fluorescent tube must be heated. In the "preheat" style of fluorescent lamp, these filaments are heated red-hot for a few seconds by sending current directly through them. There are two pins at each end of the tube and current is sent to the filament through one pin and extracted through the other pin. Once the filaments are hot enough, the lamp turns off this current flow and tries to send current through the tube itself. If the discharge starts, the discharge is able to keep the filaments hot enough to emit electrons continuously. But if the discharge fails to start, the filaments are heated some more to try to release enough electrons to initiate the discharge.

The "igniter's" job is to preheat the filaments for a few seconds and then to test the main discharge. If you see no red glow from the filaments at each end of the tube or you see no attempt by the igniter to start the main discharge, then the igniter should be replaced. It could also be that the tube itself is bad--that its filaments have burned out. If you see only one end of the tube glowing red or you see the igniter trying repeatedly to start the discharge, the tube is probably bad. I'd suggest replacing both the igniter and the tube and seeing if that fixes the problem. The only other component of the lamp, other than wiring, is the ballast--the device that controls the amount of current flowing through the discharge. It, too, could be bad.

Fluorescent light bulbs flicker rapidly because they operate directly from the alternating current in the power line. The light that you see is emitted by a coating of phosphors on the inside surface of the glass tube. These phosphors receive power as ultraviolet light and emit a good fraction of that power as visible light. The ultraviolet light comes from an electric discharge that takes place in the mercury vapor inside the tube. Since this electric discharge only functions while current is passing through the tube, it stops each time the current in the power line reverses. Thus, with each reversal of the power line, the discharge ceases, the ultraviolet light disappears, and the phosphors stop emitting visible light. So the tube flickers on and off. However, the alternating current in the United States reverses 120 times a second in order to complete 60 full cycles each second. The fluorescent lamps flicker 120 times a second. Even the very best computer monitors don't refresh their images that frequently because our eyes just don't respond to such rapid fluctuations in light intensity. In short, you can't see this flicker with your eyes. If you get eye fatigue from fluorescent lamps, it's the color or intensity of the light that's bothering you, not the flicker. It's just too fast to affect you.

It depends on how bright the light is an how long you are exposed to it. If it is simply a normal lamp, coated with some filter that absorbs all the visible light, then it is no worse than having the visible light around. It will be a very dim ultraviolet light. However, if it is a serious ultraviolet lamp, emitting several watts or even tens of watts of ultraviolet light, then it is not a great toy. Long wavelength UV is less dangerous than short wavelength UV, but neither is great. Sunlight itself contains a far amount of both long and short ultraviolet. Fortunately for us, the small amount of ozone gas in the earth's upper atmosphere absorbs much of the short wavelength UV. But long exposure to sunlight is dangerous, too.

Most "neon" lamps are mercury lamps with a colored phosphor coating on the inside. However the true neon lamp (that special red glow) is really neon gas glowing directly. Take a close look at an advertising lamp that contains a variety of colors. The mercury/phosphor ones will seem to emit light from their frosted glass walls. You are seeing the phosphors glowing. But the real neon lamp will emit light from its inside. The glass will be clear and you will see the glow originate in the gas itself.

Each atom has certain wavelengths of light that it is particularly capable of absorbing and emitting. For mercury, that special wavelength is about 254 nanometer (ultraviolet). For sodium, it is about 590 nanometer (orange-yellow). If you send a photon of the right 590 nanometer light at a sodium atom, there is a good chance that that atom will absorb it, hold it for a few billionths of a second, and then reemit it. The newly reemitted light will probably not be traveling in the same direction as before. Now if you have a dense gas of sodium vapor and send in your special photon of light, that photon will find itself bouncing from one sodium atom to another, like the metal ball in a huge pinball game. The photon will eventually emerge from the gas, but not before it has traveled a very long distance and spent a long time in the gas. It was "trapped" in the sodium vapor. This radiation trapping makes it hard for high-pressure gas discharges to emit their special wavelengths because those wavelengths of light become trapped in the gas.

Some ultraviolet fluorescent tubes are simply the mercury discharge tubes (as in a normal fluorescent tube) but without any phosphor coating on the inside of the tube and with a quartz glass tube that transmits 254 nanometer light. In such a bulb, the 254-nanometer light emitted by mercury vapor in a discharge is emitted directly from the tube without being converted into visible light. A filter somewhere in the system absorbs the small amount of visible light emitted by a low-pressure mercury discharge. For the longer wavelength black light used in most applications, other gases that emit lots of 300-400 nanometer light are used. Again, these tubes have no phosphor coatings to convert the ultraviolet light into visible light. One other way to make longer wavelength black light is to use a mercury discharge but to coat the inside of the tube with a phosphor that fluoresces ultraviolet light between 300 and 400 nanometer.

A fluorescent lamp consists of a gas-filled glass tube with an electrode at each end. This lamp emits light when a current of electrons passes through it from one electrode to the other and excites mercury atoms in the tube's vapor. The electrons are able to leave the electrodes because those electrodes are heated to high temperatures and an electric field, powered by the electric company, propels them through the tube. However, the light that the mercury atoms emit is actually in the ultraviolet, where it can't be seen. To convert this ultraviolet light to visible light, the inside surface of the glass tube is coated with a fluorescent powder. When this fluorescent powder is exposed to ultraviolet light, it absorbs the light energy and reemits some of it as visible light, a process called "fluorescence." The missing light energy is converted to thermal energy, making the tube slightly hot. By carefully selecting the fluorescent powders (called "phosphors"), the manufacturer of the light can tailor the light's coloration. The most common phosphor mixtures these days are warm white, cool white, deluxe warm white, and deluxe cool white.

The only other significant component of the fluorescent lamp is its ballast. This device is needed to control the current flow through the tube. Gas discharges such as the one that occurs inside the lamp are notoriously unstable--they're hard to start and, once they do start, tend to become too intense. To regulate the discharge, the ballast controls the amount of current flowing through the tube. In most older lamps, this control is done by an electromagnetic device called an inductor. An inductor opposes current changes and keeps a relatively constant current flowing through the tube (although that current does stop and reverse directions each time the power line current reverses directions -- 120 times a second or 60 full cycles, over and back, in the United States). Some modern fluorescent lamps use electronic ballasts--sophisticated electronic controls that regulate current with the help of transistor-like components.

They absorb the light and light energy by transferring electrons from low energy valence levels to high-energy conduction levels. These electrons wander about inside the phosphors briefly, losing energy as heat, and then fall back down to empty valence levels. Since they have lost some of their energy to heat, the light that they emit has less energy than the light they absorbed. Incoming ultraviolet light is converted to outgoing visible light.

For an atom to determine that it cannot make a particular transition (that its electron cannot move from one particular orbital to another), it must first "test the water". The atom effectively tries to make particular transition but finds that this transition is not possible. However, if the atom experiences a collision during the test period, the atom may "accidentally" undergo the forbidden transition. It is as though the atom was prevented from canceling the experiment.

The intensity of a normal fluorescent light bulb is determined by how many times each second (1) a mercury atom can absorb energy in a collision and emit a photon of ultraviolet light and (2) a phosphor particle can absorb a photon of ultraviolet light and emit a photon of visible light. The first rate depends on how much current and electrical power can flow through the tube, which in turn depends on (A) the geometry of the tube and (B) the density of mercury vapor inside. As for (A), the long, thin tube seems to be the best geometry choice for a low voltage (120V) tube, producing a certain amount of ultraviolet light per cubic centimeter of volume. The longer or fatter the tube, the more electrical power it will require and the more ultraviolet light it will produce. As for (B), at room temperature, the density of mercury vapor is just about right. In very cold weather, the density drops quite low and the bulb becomes dim (thus fluorescents are not recommended for outdoor use in cold climates). Finally, the second rate (conversion to visible light) depends on the coating of phosphors on the inside of the tube. A tube that is too fat will send too much ultraviolet light at the phosphors and they will become inefficient. So a long thin tube is a good choice again. Each region of tube surface converts the light from a relatively small volume of mercury gas. Overall, the intensity of the bulb scales roughly with the volume of the tube. Big tubes emit more light than little tubes. One of the challenges facing fluorescent lamp manufacturers is in making small tubes emit lots of light. To replace an incandescent lamp with a miniaturized fluorescent, that miniaturized fluorescent must emit lots of light for its size. They're getting better every year, but they aren't bright enough yet.

A true neon light tube has completely clear (no color, no phosphor) glass surrounding a thin gas of neon atoms. When current runs through that gas, the neon atoms emit red light. In "neon tubes" that emit colors other than red (green, pink, orange, yellow, etc.), there is a layer of phosphor on the inside surface of the glass and mercury vapor inside the tube. These fluorescent tubes probably don't contain any neon at all. You can see the light coming from the phosphor coating. In a true neon tube, you can see the light coming from the gas itself, well inside the glass tube.

While fluorescent light fixtures do emit magnetic fields, those fields are far too weak to affect magnetic media. Any electric current produces a magnetic field, even the current flowing through the gas inside a fluorescent tube. However, that field is so weak that it would be difficult to detect. Nearby iron or steel could respond to that weak magnetic field and intensify it, but the field would still be only barely noticeable. The only strongly magnetic component in a fluorescent fixture is its ballast coil. The ballast serves to stabilize the electric discharge in the lamp and relies on a magnetic field to store energy. However, the ballast is carefully shielded and most of its magnetic field remains inside it.

As for affecting diskette magnetic media, that's extraordinarily unlikely. Even if you held a diskette against the ballast, I doubt it would cause any trouble. Modern magnetic recording media have such high coercivities (resistances to magnetization/demagnetizations) that they are only affected by extremely intense fields.

Yes. Tanning appears to be your skin's response to chemical damaged caused by ultraviolet (high energy) light. Each photon of ultraviolet let carries enough energy to break a chemical bond in the molecules that make up your skin. Exposure to this light slowly rearranges the chemicals in your tissue. Some of the byproducts of this chemical rearrangement trigger a color change in your skin, a change we call "tanning". Any source of ultraviolet light will cause this sequence of events and produce a tanning response. However, the different wavelengths of light have somewhat different effects on your skin. Long wavelength ultraviolet (between about 300 and 400 nanometers) seems to cause the least injury to cells while evoking the strongest tanning response. Short wavelength ultraviolet (between about 200 and 300 nanometers) does more injury to skin cells and causes more burning and cell death than tanning. However, all of these wavelengths have enough energy to damage DNA and other genetic information molecules so that all ultraviolet sources can cause cancer.

I'll have to guess at this one. If the lamps you are talking about are mercury vapor, then they contain a reservoir or droplet of liquid mercury. If shaking these lamps would cause the mercury to flow out of the cooler reservoir and into hotter regions of the bulb, the mercury would boil and raise the pressure inside the lamp. The current passing through the lamp would increase and the bulb would get very bright. It would also get hotter and hotter, so its pressure would rise still further. Eventually the pressure would become so high that the bulb would explode.

Most modern commercial and industrial floodlights are fluorescent lamps. Fluorescent lamps are so much more energy efficient than incandescent lamps that they quickly pay for their higher cost by saving electricity. Fluorescent lamps also last much longer than incandescent lamps, particularly if they are left on for long periods of time. Fluorescent lamps age most during their start-up cycles. Even around the house, fluorescent floodlights are becoming popular. Fluorescent lamps using about 150 W of power are as bright as incandescent lamps using 500 W. Both are bright, but one is much more energy efficient.