March 31, 1995
One Minute Papers - Questions and Answers
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.
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).
If I stood in front of Mr. Jones' laser, would I die?
His laser emits an incredibly intense pulse of remarkably short duration. The amount of energy in each pulse is very small; less than the energy in a few grains of sugar. As a result, the laser is not very dangerous because it cannot do deep damage. It can only injure the first few layers of skin if it hits you. It can, however, do eye damage so you must be careful not to look into a beam from this or any other laser. Your eyes are particularly sensitive to light and your retina are only a few cells thick. Surface damage to your retina is bad news.
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.
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.
Wouldn't a laser in laser surgery cut straight through the organ being worked on?
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.
Can you explain how laser disc players work?
That is the topic of the lecture on April 10.
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.
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.
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.
Why is a semi-transparent mirror better than metal and how does it work?
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.
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 not possible to synchronize the charged particles that emit visible light. You must use stimulated emission if you want to create coherent light.