March 29, 1995

One Minute Papers - Questions and Answers

Can you get a tan from an ultraviolet light bulb?

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.

Are flood lights incandescent or fluorescent? Why are they so bright?

Most modern commercial and industrial flood lights 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 flood lights 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.

As a kid, we'd shake street lights. They'd get real bright and then explode. Then we'd run away. Why'd they get brighter and explode?

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.

How does that remote control turn on/off everything in the room at the touch of a finger?

The infrared remote control works the same way as all infrared remotes work: it sends an encoded message to a small computer that controls the flow of current to the overhead lights. The current control functions just like a normal light switch, although it is operated by the computer rather than your finger. When the switch is opened, the fluorescent overhead lights turn off. When the switch is closed, the lights go through their normal start-up sequence (heating their electrode filaments and then striking the main discharge across each tube). The infrared remote controller emits a series of high-frequency light pulses. It sends 1's and 0's (binary information) using an FM (frequency modulation) technique. I don't know the exact frequencies, so here is a good guess: To send a "1", it emits a quickly spaced series of pulses (about 500,000 pulses per second). To send a "0", it sends a more widely spaced series of pulses (about 300,000 pulses per second). The receiver looks for these rapidly blinking light patterns. Since virtually nothing else in the room emits such rapid pulses of infrared light, it can easily find the remote controller's emission from across the room. It then reads the encoded message and decides what to do with the room lights.

Why doesn't light go through the other side of a water droplet, refracting as it goes through, rather than reflecting back?

Actually, 96% of the light hitting the "other side of a water droplet" does pass out of the droplet. What you see in the rainbow is the 4% that reflects back from the far side of the water droplet. If all of the light reflected, the rainbow would be much brighter.

If white color has a reflection close to one, what role does shininess or dullness play?

Just because two materials both reflect all of the light that strikes them doesn't mean that they look the same. When you send a flashlight beam at a white surface, you can see that reflected light from all directions. When you send the flashlight beam at a mirror surface, you can only see the reflected light from one particular angle. Both the white surface and the mirror surface reflect virtually all of the light that hits them. A shiny white surface is different from a dull white surface because a shiny white surface has a small amount of mirror character to it: you can see the whiteness from any direction but there is also a mirror aspect that you can only see from certain angles.

Is having a black light in your room dangerous?

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.

How do phosphors change the light from ultraviolet to visible?

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.

What happens when a fluorescent lamp flickers during start-up but doesn't fully light?

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.

Is a neon light actually a mercury/phosphor tube?

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.

Where does the extra energy go after ultraviolet light goes through the phosphor coating? Is it lost as heat?

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.

Why do mercury lamps without phosphors emit visible light at high pressure? What are the "forbidden" transmissions?

At low pressure, a mercury lamp emits mostly 254 nm 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 nm light out of the lamp. Whenever one of the atoms emits a particle of 254 nm 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 nm 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.

Does the size of the bulb affect its intensity?

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 to 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 requires that that miniaturized fluorescent emit lots of light for its size. They're getting better every year, but they aren't bright enough yet.

Why do many fluorescent lamps blink before they come on?

The lamp first heats the filaments in its electrodes red hot (so that they begin to emit electrons), then it 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 to start (heating first, then trying to start the discharge). If may blink several times before it gets enough of a discharge to start so that the discharge is able to sustain itself (each electric charge must recreate itself at least once during its trip across the tube).

How did we see the light spectrum of the bulbs in class? Did we use some type of instrument?

Yes. We used a device called a "diffraction grating" to separate the various wavelengths of light. A diffraction grating is a surface that has been scratched with a pattern of very closely-spaced lines. Light reflecting from the different scratches interferes so that different wavelengths of light end up traveling in slightly different directions. The video camera sees the light as coming from different directions (which is why you see the bulb appear several times, each with its own color). Diffraction gratings used to be rare but they now appear on countless toys. Any time you look at a surface and see it break the light striking it up into various colored flashes of light, you are looking at a diffraction grating.

How does suntan lotion work to prevent ultraviolet rays from damaging your skin?

Suntan lotion (or rather sunscreen) is a chemical whose molecules absorb ultraviolet light and turn its energy into heat. Like fluorescent compounds, these molecules absorb ultraviolet light strongly. But unlike fluorescent compounds, the sunscreen molecules do not reemit any light. They convert all of the ultraviolet light energy into heat, which does no damage to your skin.

What actually causes a fluorescent bulb to burn out?

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.

Why do some car fog lights use yellow light and others use white?

I don't know this so I'll have to guess. Yellow or red light scatter less strongly than blue light from small water droplets. Thus if you want your headlights to pierce through the fog and see beyond it, you are probably better off with the longer wavelength yellow or red light than with white light (which contains lots of green and blue). However, your eyes are not very sensitive to red light so yellow is a better choice than pure red.

Why do you have blocks around the base of the 2 lights off to the side?

During the demonstrations of the two high pressure lamps, I had blocks around them at their bases. Those blocks were merely to support some opaque screen so that I could prevent their light from shining directly in people's eyes. In the end, I decided not to use the screens.

How do "forbidden transitions" become less forbidden as pressure builds?

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.

How does radiation trapping work?

Each atom has certain wavelengths of light that it is particularly capable of absorbing and emitting. For mercury, that special wavelength is about 254 nm (ultraviolet). For sodium, it is about 590 nm (orange-yellow). If you send a photon of the right 590 nm 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.

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