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Recently, I got a question about compact disks. The reader wanted to know what they are made of and what the difference is between a CD and a DVD. These are good questions, so I went to one of my favorite research sites for answers. If you don't know about the How Stuff Works site (http://www.howstuffworks.com), get ready to waste an afternoon. It's a trivia-meister's dream and it's easy to lose track of time.
Anyway, I tried not to get distracted. I learned that most of a CD is made of clear polycarbonate plastic. The plastic has tiny indentations (or pits) molded into it. The data is arranged as a long string that begins in the center and spirals to the outside. The laser reads a CD from the underside of the CD and because the distance from the surface and a pit is different, you can store binary 1s and 0s (on or off), which are the building blocks of a digital signal.
Above the polycarbonate plastic layer is a layer of aluminum. On top of that is a layer of acrylic, and then the label, which is often silk-screened directly on the acrylic. As some people remember, originally compact discs were just used for audio recordings. Later the same physical size was used for CD-ROMs to store data as well. CD-ROMs are mass produced using a process called "stamping" unlike CD-Rs or CD-RWs which are recorded or "burned" by computer users.
A DVD is much like a CD except that it can hold more data. Like a CD, a DVD is made of polycarbonate that has tiny pits in it that a laser can read. A CD holds about 640MB of data, whereas a single-sided single-layer DVD can hold 4.7GB. A single-sided double layer DVD can hold 8.5GB and a double-sided double layer DVD can hold 17GB. The reason a DVD can hold more data is because the pits are spaced closer together and it can have multiple layers (up to two on each side). DVDs also don't use up as much space for error correction as CDs do.
Both CDs and DVDs have made storing lots of data far easier than it used to be. In fact, I recently got a new computer, and for the first time I opted not to get a floppy disk drive at all.
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