The laser you're using is a neodymium-YAG laser. It uses a crystal of YAG (yttrium aluminum garnet), a synthetic gem that was once sold as an imitation diamond, that has been treated with neodymium atoms to give it a purple color. When placed in a laser cavity and exposed to intense visible light, this crystal gives off the infrared light you describe. You can't see this light but, at up to 600 watts, it is actually incredibly bright. You don't want to look at it or even at its reflection from a surface that you're machining. That's because the lens of your eye focuses it onto your retina and even though your retina won't see any light, it will experience the heat. It's possible to injure your eyes by looking at this light, particularly if you catch a direct reflection of the laser beam in your eye.

In all likelihood, the manufacturer of this unit has shielded all the light so that none of it reaches your eyes. If that's not the case, you should wear laser safety glasses that block 1064 nm light. But it's also possible that the irritation you're experiencing is coming from the burned material that you are machining. Better ventilation should help. High voltage power supplies, which may be present in the laser, could also produce ozone. Ozone has a spicy fresh smell, like the smell after a lightning storm, and it is quite irritating to eyes and nose.

You are probably referring to a device developed at the BC Cancer Research Center in Vancouver, British Columbia and now available commercially from Xillix Technologies. A scientist from that research center gave me the following description of their technique.

The instrument is based on the discovery that most tissues when illuminated by blue or UV light emit a natural fluorescence spectral signature known as autofluorescence. This fluorescence signature is the sum of the emission of the various biochemical fluorphores present in the tissue. If the tissue chemical or physical structure changes, then the spectral signature changes. By exploiting differences in the spectral signature between cancerous and healthy tissue one can create an imaging device that can "see" the difference in the color of the autofluorescence of the tissue and detect changes that may indicate the presence of cancer. The sensors used to see the low levels of fluorescence light employ similar technology to military night vision devices. Once areas of change are located and confirmed by analysis of a biopsy sample treatment can begin. This technique is primarily useful for early stage cancers that are not visually apparent to a physician.

A laser printer uses a single diode laser that's scanned across the surface of the photoconductor drum by a rapidly turning, multifaceted mirror. These diode lasers are very similar the ones used in laser pointers or supermarket barcode readers. The multifaceted mirrors are typically octagonal prisms that are aluminized to make them highly reflecting and spun by a motor. The laser beam bounces off the spinning mirror and its reflection sweeps across the photoconductor. Modulating the current supplying power to the diode laser causes its brightness to fluctuate so that it writes information on the surface of the photoconductor.

They do use mirrors. When you bounce a laser beam from a mirror, any small change in the mirror's orientation can cause a large change in the beam's final destination. Simple laser light shows bounce lasers from low-mass mirrors that are mounted on elastic membranes. As those membranes are driven into motion by sound waves, the mirrors tip and turn and the laser beams move around in beautiful patterns on a distant screen or wall. In laser light shows that produce specific shapes and images, the mirrors that steer the laser beams are driven by high-speed electromagnetic mechanisms that can change a mirror's angle dramatically in thousandths of a second. With several of this electromagnetically controlled mirrors working together and guided by a computer, the beam can be steered to draw complicated shapes on a screen or other surface.

The classic technique for making a hologram begins with splitting the light from a laser into two parts. Part of the laser light is used to illuminate a scene while the other part is used to illuminate a piece of film placed in front of the scene. Actually, the film is exposed to light from two sources: (1) the second part of the laser beam and (2) a portion of the first part of the laser beam that the objects reflect toward the film. Lights from these two sources don't simply add when they reach the film; they interfere with one another. Laser light is unusual in that it is coherent light--a giant wave consisting of numerous identical particles of light. When the wave from the laser and the wave reflected from the objects meet at the film, they interfere. When the crest of one wave joins the crest of the other wave, the two waves form an extra large crest--constructive interference. But when the crest of one wave joins the trough of the other wave, the two waves cancel and produce essentially nothing--destructive interference. Because of this interference, the film ends up recording not only the intensity information that we associate with normal photography; it also records phase information that is an important aspect of waves. This phase information indicates where crests and troughs in the wave occurred. Because the hologram contains both kinds of information, it allows a viewer to see things that they would not see in a simple photograph.

To make a hologram, you should take a laser and split its light into two unequal portions with the help of a laser beam-splitter (or even a glass slide). The laser should operate at only a single wavelength, so that its light is highly coherent, and it should have a coherence length much longer than any distance in the scene--two requirements that are met by most common continuous-wave lasers, including laser pointers and basic helium-neon lasers. Send the stronger portion of the laser beam through a diverging lens and allow it to illuminate a scene that is otherwise in complete darkness. Light reflected from this scene should reach the film holder in which the hologram will be made. Send the weaker portion of the laser beam through another diverging lens and allow it to illuminate the film holder from the scene side. For best results, the light reflected from the scene on the film holder should be about as bright as light from this second beam.

Now place fine-grained black and white film in the film holder. Be sure that the film is sensitive to the laser light--some black and white films aren't sensitive to red light. Allow light to strike the film for long enough to expose it. Finally, develop the film and observe the developed film while it's illuminated from behind with laser light that has been spread out by a diverging lens. You should see the original scene as a three-dimensional image.

Unfortunately, there is one detail I've omitted until now. To make sure that the phase information is properly recorded, you must be sure that nothing moves by even a fraction of a wavelength of laser light during the entire exposure period. That's a very demanding requirement. Vibrations are everywhere and they will spoil the hologram. If you want this technique to work, you'll have to isolate everything--the laser, the optics, the scene, and the film--from vibrations. In a laboratory, this vibration isolation is done by floating a massive optics table on a cushion of air. All of the objects involved in making the hologram are rigidly attached to this table so that they can't move. As an alternative, you can put all the objects for the hologram on as rigid and massive a surface as you can find and support that surface on a thick layer of foam rubber. Make the holograms at night when there is little traffic of any sort around and be sure that nothing is jiggling about nearby that might shake the floor even a little bit. If you're careful, you ought to be able to create a hologram with such an arrangement.

Lasers use excited atoms or atom-like systems to amplify light. Putting mirrors around such excited atoms or atom-like systems allows them to amplify their own light until the laser is emitted vast numbers of identical light particles or "photons." To burn something with laser light, there must be a great many excited atoms or atom-like systems and they must be very efficient at amplifying light. Probably the easiest to build powerful laser is a carbon dioxide laser. This laser uses an electric discharge in a mixture of nitrogen and carbon dioxide gas to produce excited carbon dioxide molecules. These molecules amplify infrared light at a wavelength of 10.2 microns extremely efficiently, so that a laser consuming about 1000 watts of electric power can emit approximately 100 watts of infrared light. That's enough power to burn things very quickly. Even more powerful carbon dioxide lasers are used in industry to cut and machine metals, including thick steel plates. But while they are surprisingly simple to build and operate, given the right components, carbon dioxide lasers require dangerous high voltage power supplies. There were many physics graduate students electrocuted in the 1960's while tinkering with homemade carbon dioxide lasers.

No. The reason that you can see a very intense laser beam as it passes through the air is that light can scatter off of dust particles and air molecules. When it does, some of the laser light is sent toward your eyes and you see the light coming toward you from the laser beam's path. But if there is no air in the path of the laser beam, the light will travel without scattering and you won't see the path at all.

If you are referring to a system that displays illuminated line drawings on a wall that move with the music, then building one is easy. You need a small isolated speaker--just the electronic device, not a whole speaker unit--that you can connect to the music amplifier. Place an elastic membrane over that speaker--a stretched sheet of thin rubber from a latex glove should work well. Then glue a tiny, front-surface mirror to that rubber membrane, choosing a point that is about midway between the middle of the speaker and its edge. A front surface mirror is one that is shiny on its top, so that light doesn't have to go through glass before reflecting. A broken fragment of mirror, about 3 mm on a side, should work. Finally, shine the beam of a laser pointer onto the mirror and begin to play music through the speaker. The mirror will move with the music and the reflected laser beam will form pretty patterns on the wall.

While most diode lasers operate at several frequencies simultaneously, it's possible to make lasers that function at only one frequency. In fact, such "single mode" diode lasers are extremely stable light sources and the basis for much current research in optical science. For example, the recent observations of Bose condensation in vapors of alkali metal atoms were made with the help of single mode diode lasers.

The phrase "single mode" refers to a single longitudinal wave that travels back and forth through the device while it is operating. This single wave has one frequency and one wavelength. It is selected from other possible waves through the use of interference effects. For the wave to be stable inside the laser cavity (the laser is bounded at each end by a mirror, thus forming an optical cavity), the cavity's length must be an integer or half integer multiple of the light's wavelength. While that criterion alone will allow several possible waves to form, coupling a second cavity to this laser cavity further restricts the wave so that only a single wave can operate inside the laser. The diode laser will then have only a single mode of operation and will emit a single frequency of light.

Lasers use systems with excess energy to amplify light. These systems, typically atoms or atom-like structures in solids, are in excited states--they have more than their minimum amounts of energy. An excited system can get rid of its excess energy in many different ways, but certain systems tend to emit the excess energy as photons--particles of light. While an excited system will emit a photon spontaneously if you wait long enough, it can also duplicate a passing photon if that passing photon has the proper characteristics. Most importantly, the excited system must be naturally capable of emitting the passing photon spontaneously--the passing photon's wavelength and travel path must be such that the excited system is able to duplicate it.

This duplication effect makes it possible to amplify light. When a single photon passes by a number of identical excited systems, those systems may duplicate the photon many times so that many identical photons emerge. This phenomenon is the basis for laser amplifiers. When one of the photons emitted spontaneously by the excited systems is deliberately sent back and forth through those systems with the help of mirrors, the laser amplifier becomes a laser oscillator--it both initiates and amplifies the light. The light that ultimately emerges from the laser oscillator or amplifier differs from normal light because the laser light consists of many identical photons. They all have identical wavelengths (colors) and follow identical paths through space. They also exhibit dramatic wave effects, particularly interference.

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.

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.

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.

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.

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.

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.

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).

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.

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.

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.