As a kid, we’d shake streetlights. They’d get real bright and then explode. Then…

As a kid, we’d shake streetlights. 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.

Is having a black light in your room dangerous?

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.

Why do mercury lamps without phosphors emit visible light at high pressure? What…

Why do mercury lamps without phosphors emit visible light at high pressure? What are the “forbidden” transitions?

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.

Can you get a tan from an ultraviolet light bulb?

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.

I’ve heard (from observations recorded in an office environment) that fluorescen…

I’ve heard (from observations recorded in an office environment) that fluorescent light bulbs “emit” their energy at a certain frequency. If this frequency is at or below the rate at which our eyes blink/scan, this will cause eye fatigue and other health “problems.” What would be the best light system for the office environment?

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.

Why does a fluorescent bulb sometimes appear blue, especially right before it bu…

Why does a fluorescent bulb sometimes appear blue, especially right before it burns out?

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.

Do fluorescent light fixtures emit magnetic fields? If so, would they be intense…

Do fluorescent light fixtures emit magnetic fields? If so, would they be intense enough to affect diskette magnetic media?

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.

We have some problems with a “fluorescent lamp igniter”, the device that turns…

We have some problems with a “fluorescent lamp igniter”, the device that turns on the lamp. I would like to know what is necessary for the fluorescent lamp to turn on?

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.