Month: December 1969

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.

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 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.

Do neon lights have glass that is not colored, but has phosphors that emit a particular color?

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.

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.

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.

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.

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

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.