text: To understand the answer better, I have prepared a small course on CD's for you. \n\n--How the CD is constructed--\nA CD is a fairly simple piece of plastic, about four one-hundredths (4/100) of an inch (1.2 mm) thick. Most of a CD consists of an injection-molded piece of clear polycarbonate plastic. \n\nDuring manufacturing, this plastic is impressed with microscopic bumps arranged as a single, continuous, extremely long spiral track of data. Once the clear piece of polycarbonate is formed, a thin, reflective aluminum layer is sputtered onto the disc, covering the bumps. Then a thin acrylic layer is sprayed over the aluminum to protect it.\n\n--The spiral on a CD--\nA CD has a single spiral track where data is burned on, circling from the inside of the disc to the outside. It is extremely small - approximately 0.5 microns wide, with 1.6 microns separating one track from the next. (A micron is a millionth of a meter.)\n\n--How things are written onto a CD--\nInformation are burned into the plastic of a CD in the form of small pits, or bumps. The bumps that make up the round track are each 0.5 microns wide, a minimum of 0.83 microns long and 125 nanometers high. (A nanometer is a billionth of a meter.)\n\nThe incredibly small dimensions of the bumps make the spiral track on a CD extremely long. If you could lift the data track off a CD and stretch it out into a straight line, it would be 0.5 microns wide and almost 3.5 miles (5 km) long! \n\nA CD drive has a laser beam which shines onto the track of the CD and picks up the bumps. The bumps are converted to information that the computer or CD player understands.\n\nIf you want more detailed information, visit http://electronics.howstuffworks.com/cd.htm\n\nHope I could help...