Optically, they are identical. However, the laser disc player uses an analog recording technique (non-digital) to recreate the video signal. I think that the lengths of ridges (or perhaps pits) in the aluminum surface are used to control the analog signal strength.
It doesn’t. It reads only a tiny portion of the disk at any given moment. The disk spins and the reading system slowly works its way from the center of the disk towards its edge, following a spiral path around the disk.
A diode laser will only emit light (lase) when current flows through it in the proper direction. It is, after all, a diode and only conducts current in one direction. But small fluctuations in current do affect the light emission. If you run a modest current through a laser diode, so that it emits a steady stream of light, and then begin to modulate that current up and down slightly, the light emitted by the laser will modulate up and down slightly, too. In this manner, you can send sound or other information over a laser beam. This technique is useful as a private means of communicating over long distances. Only someone who can “see” the blinking laser beam can detect the information that it contains.
A laser diode resembles a light emitting diode, in which electrons flowing across a p-n junction (in a diode) find themselves in conduction levels of the p semiconductor, with lots of excess energy. These excited electrons give up their excess energy by emitting light and they drop down into empty valence levels with much less energy. In a laser diode, the region in which this energy release occurs is a very narrow channel with mirrored ends. Instead of emitting their light spontaneously, the electrons experience stimulated emission. Light bounces back and forth between the ends of the channel and is amplified as it passes new excited electrons. Because all of the light produced by a laser diode emerges from one end of this very narrow channel, it experiences severe diffraction and spreads out into a wide, cone-shaped beam. To convert this cone of light into a narrow beam, a converging lens is usually attached to the diode laser’s housing and this lens bends the beam into a fine pencil of light. Most laser diodes operate in the red or infrared portion of the spectrum, although some laser diodes that emit blue light have recently been developed.
In principle, the binary numbers could be written as the presence or absence of ridges (i.e. a 1000 nanometer long ridge could be a 1 while a 1000 nanometer long flat area could be a 0). However, this technique has technical problems. The main problem is that the number “0” would be a long flat region (16 adjacent flat regions would be one 16000 nanometer flat region). If the flat region became too long, the CD wouldn’t be able to follow the track any more. So an encoding scheme is used to make sure that ridges and flat areas are never too long. They use a length-encoding scheme, where ridges of different lengths correspond to a short group of binary bits. Furthermore, a very extensive error correcting arrangement makes sure that the music can be read even if a great many bits are unreadable. About 25% of the CD’s surface is dedicated to this error correcting information.
Digital audiotapes have been around for a few years. These tapes store sound as digital information on a tape. Because of the digital recording and playback, the reproduction is almost perfect. The digital process involves an enormous amount of information each second; too much to be recorded in the conventional method used in cassette tapes. Instead, I think that a helical technique is used, in which information is written as diagonal stripes across the length of the passing tape. By writing a closely spaced series of these stripes, the DAT (digital audio tape) player uses much more of the tape’s surface than a standard cassette and stores much more information on that surface. I doubt that DAT tapes will replace CD’s because CD’s are so easy to mass-produce. DAT tapes must be recorded one at a time.
The digitization process does introduce some distortions into the sound signal, including aliasing (confusion about high frequencies) and quantization error (round-off errors in recording the softest sounds). However, these distortions should be so small or at such high frequencies that they should be inaudible. Still, there are always some audiophiles who can hear (or claim to hear) these imperfections.
In forming a real image, a camera lens behaves symmetrically, taking light reaching it from above its central axis and projecting that light onto a spot below its central axis. But in forming a virtual image, a magnifying lens merely redirects the light subtly to have it appear to come from a point nearer or farther than the original object. You still see the object as it was (right-side up) but moved toward you or away from you.
A virtual image is always located behind the optic (lens or mirror) that creates it. Thus when you look into a magnifying glass, eyepiece, or a make-up mirror, you see light that appears to come from beyond the optic that creates the image. You can’t touch the virtual image or put your hand in the pattern of light that you seem to see. The virtual image can appear to come from just behind the optic or from a great distance behind that optic. It depends on how things are arranged. As you lift a magnifying glass off the surface of a newspaper, the virtual image of the newspaper starts just behind the glass and slowly moves back away from the glass. As the distance between the magnifying glass and newspaper approach the magnifying glass’s focal length, the virtual image moves away to an infinite distance behind the glass. After that, there is no longer a virtual image at all. Instead, a real image begins to appear on the other side of the magnifying glass.
A spoon forms an inverted real image of you when you look into the concave (hollow) side. This real image is located a few centimeters in front of the spoon, where you can touch it with your finger or insert a small piece of paper into it. Try it, you will see the pattern of light appear on the paper sliver. A spoon forms a right side up virtual image of you when you look into the convex (bowed outward) side. This virtual image is located a few centimeters behind the spoon. You appear very small because it is a small virtual image that you are looking at. You cannot touch this virtual image.