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Lasers
Page 1 of 3 (21 Questions and Answers)

394. How does laser surgery work?
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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?
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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?
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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?
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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?
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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?
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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)?
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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.

401. Why is a semi-transparent mirror better than metal and how does it work?
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Metal mirrors usually absorb about 5% of the light that strikes them. Thus a fully reflective metal mirror, with a thick layer of aluminum, silver, gold, or some other metal, will typically only reflect about 95% of the light. A partially reflective metal mirror, with a very thin layer of metal, might reflect 50% of the light, transmit 45% of the light, and absorb 5%. That 5% absorption is terrible in a laser because the metal layer will heat up and fall apart. Instead, dielectric (insulator) mirrors are created. These mirrors used layer after layer of perfectly clear insulators (usually metal oxides and metal fluorides) to reflect light. Each time light moves from one of these layers to the next, its speed changes and part of it reflects. The thicknesses of the layers are carefully controlled so that the desired wavelengths are reflected in just the right amounts. Since the layers absorb no light, any light that is not reflected is transmitted. A dielectric mirror might reflect 50% of the light, transmit 50% of the light, and absorb 0%. Since they absorb no light, dielectric mirrors do not heat up in use and work well with even very high-powered lasers.

402. Wouldn't a laser in laser surgery cut straight through the organ being worked on?
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Laser light, like any other light, only travels so far in a material that absorbs it. In surgery, the wavelength of light is chosen so that it is absorbed near enough to the source that it doesn't damage tissue far from the source. The laser vaporizes nearby material but doesn't burn holes through people. If the surgeon paused for a long time, the hole being cut would gradually get deeper. But normally, the depth of the cut isn't very great.

574. How do some paints and stickers glow in the dark? — DD, Sandy, UT
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Glow in the dark paints and materials contain molecules that are able to store energy for long periods of time and then release that energy as light. To understand how this delayed emission works, let's examine the interactions of molecules and light. The electrons in any molecule are normally arranged in what is called the "electronic ground state," an arrangement that gives those electrons the least possible energy. However, the electrons in a molecule can also be arranged in one of many "electronically excited state," in which they have more than the minimum energy. Whenever a molecule is exposed to light, its electrons may rearrange and the molecule may find itself in one of the electronically excited states. If that occurs, the molecule will have absorbed a particle of the light, a "photon," and used the photon's energy to rearrange its electrons.

In a typical molecule, the extra energy is released almost immediately, either as light or as the vibrational energy that we associate with heat. But in a few special molecules, this extra energy can become trapped in the molecule. When an electron shifts from one arrangement to another and the total energy of that molecule decreases, the missing energy may leave as a photon of light. But electrons behave as though they were spinning objects and in shifting between arrangements, the electron normally can't change the direction of its spin. In most rearrangements that lead to the emission of light, the electron spins remain unchanged.

However, a glow in the dark molecule is one in which there is an electronically excited state that can only shift to the ground state if one of the electrons changes its direction of spin as the photon of light is being produced. In some molecules, this process is almost totally forbidden by the laws of physics and proceeds so slowly that the molecule may wait for minutes, hours, or even days before it emits the photon and returns to its ground state. When you expose a material containing these molecules to light, its molecules become trapped in these special electronically excited states and they then glow in the dark for a long while afterward.


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