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Page 40 of 160 (1595 Questions and Answers)

391. 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 and then 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; first heating its filaments and then trying to start the discharge. The lamp may blink several times before the discharge becomes strong enough to keep the electrodes hot and sustain the discharge.

392. 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.

393. 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.

394. How does laser surgery work?
Lasers are used in medicine in a variety of ways. In surgery, lasers are used mostly as intense sources of heat. They deposit large amounts of power into small areas, vaporizing and "cooking" tissue. Because they produce very local heating, there is no much bleeding from a cut made with a laser scalpel. In some eye surgery, intense pulsed lasers are used and take advantage of the peculiar effects that happen at very high intensities. The most important of these effects is the creation of free charged particles, which reflect and absorb the laser beam. Because it creates free charged particles when it encounters a surface, an intense pulsed laser beam only penetrates a few microns into a surface. The charged particles that it creates prevent it from traveling deeper, even in a clear material. In eyes, that allows surgeons to remove outer layers of tissue without damaging inner layers or the retina beyond.

395. Is all light other than lasers incoherent?
Yes, in the sense that the only way to create coherent light is through the use of laser amplification. While it is possible to create coherent radio waves by synchronizing the motion of many charged particles, it is extremely difficult to synchronize the charged particles that emit visible light. (The one exception to this statement is a free electron laser, an exotic device that uses the beam of electrons from a particle accelerator to produce coherent light.) In general, you must use stimulated emission if you want to create coherent light.

396. What do mirrors do for lasers?
Mirrors help to create laser beams by sending light back and forth through the laser medium. They also reflect laser beams and are used to redirect laser light.

397. What is an interference pattern in lasers?
When the wave of light emitted by a laser can follow more than one path to a target, the waves taking the different paths may "interfere" with one another. If the electric field in the wave taking one path is in phase with (always pointing in the same direction as) the wave taking another path, then the two waves will help one another and they will push together on charges in the target. The amount of light reaching the target will be particularly strong. However, if the two waves arrive out of phase with one another (always pointing in opposite directions), then they will cancel one another and the amount of light reaching the target will be particularly weak. Usually a pattern of bright and dark regions appears on an extended target as the waves following different paths alternately interfere constructively (helping one another) and destructively (canceling one another).

398. What kinds of lasers are used at laser demonstrations? Why and how do they get different colors? How do you see the actual beams?
Most of the visible lasers used in light shows are gas lasers: tubes with gas discharges in them that are arranged to produce laser light. The most common gases used in these tubes are argon and krypton. Argon lasers produce green and blue light very nicely, while krypton lasers are best for intense red light. The colors come from the structures of the atoms themselves; the energies of their various electron orbitals. To see the beams, something must scatter the light. If the lasers are intense enough, Rayleigh scattering from the air is enough to make the beams visible. However, a little mist added to the air helps a lot.

399. Why are lasers harmful to your eyes?
You eyes treat the laser light as though it came from a very distant object with a very small size. As a result, your eyes focus all of the laser light to a single tiny spot on your retina because that is where light from a tiny, distant object should go. However, there is a lot of power in the laser light and when all of that power lands on only a few cells at the surface of your retina, it cooks those cells. Its very similar to what happens when you hold a magnifying glass in sunlight and create a white hot spot on a piece of wood. With powerful lasers, damage can be done to your retina very quickly.

400. Why does the laser not create a beam of light that you can see as it travels through the air to its destination (like a flashlight)?
You can only see light travel across a room if something in the air scatters that light toward you. If there is dust, smoke, or mist in the air, you will see that light pass through it. You will see a flashlight beam scattered by these particles and you will also see a laser beam. In that respect, the two kinds of light are very similar. Some laser beams are so intense that the Rayleigh scattering (the scattering that creates the blue sky) is strong enough to make the beams visible even in perfectly dust-free air. The beams shown in class are not that strong and would only be visible if something in the air scattered their light toward your eyes.
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