. How do neon lamps work? — TF, Auburn, WA
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.
. What causes things to glow in the dark? Why does phosphorus glow? Why does the glow die? — DB & EB
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.
. How does luminol work? — CW, San Antonio, TX
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.
. I have experimented with passing high voltage arcs through ionic compounds and have observed different colors when I do. An arc through salt (sodium chloride) produces a brilliant yellow light. How does this work? — JB, Lantana, FL
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.
. There is a debate amongst the teachers in our school as to what are the three primary colors. Some say Red, Green, and Blue, others say Red, Yellow, and Blue. Do you have an explanation? — RS, Farmington Hills, MI
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.
. Is there an effective shield for the EMF generated from mercury vapor ballasts? — CS, Washington, DC
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.
. Does food coloring change the color of food?
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.
. How do the spectrums of different light sources differ? For example, when you look at an incandescent bulb through a spectroscope, do you see colors other than what you see when you look at a fluorescent bulb? — EC, Tokyo, Japan
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.
. How does a black light work? — JLM, Kettering, OH
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.
. What is zero point energy? — AWG, Karachi, Pakistan
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.