. How does a strobe light work? — JM, Kettering, OH
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.
. I know that photons are particles of light—but how are photons related to the "excited" electrons in the atoms of a gas discharge?
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.
. When you were saying that even humans travel as waves (which I can picture), is this the theory behind how the people in the show Startrek are "beamed" to certain planets and back to the ship?
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.
. How do neon lights work? — MT, Cement City, MI
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.
. Why is incandescent lighting better in residential construction than metal halide, high-pressure sodium, or mercury vapor lighting systems? — JC, Halifax, Nova Scotia
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.
. What are atoms made of? — Fifth Grade Class, Knifley, KY
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.
. Do the resonant frequencies of the elements change as the magnetic fields they reside in change? Can an element such as iron be made to resonate at the magnetic field strength of the earth? — JP, Blakeslee, PA
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.
. How do neon lights work? — MT, Cement City, MI
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.
. Is light a particle or a ray? — CG
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.
. I have heard that there is a substantial cost to starting a fluorescent light fixture. When entering and exiting a room frequently, is it better to leave a fluorescent light turned on, or to turn it off when leaving each time? — GEW
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.