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To fit 74 minutes of music (that are 782,216,000 bytes) onto a disk with only 12 centimetres in diameter means that the bytes have to be fairly small.
A CD is a simple piece of plastic about 1.2 millimetres thick. It consists of an injection-molded piece of clear polycarbonate plastic. During manufacturing this plastic is impressed with microscopic bumps arranged as a single extremely long spiral track of data. Once the clear piece of polycarbonate is formed, a thin, reflective aluminium layer is sprayed onto the disk, covering the bumps. Then a thin acrylic layer is sprayed over the aluminium to protect it and the label is printed onto the acrylic.
A CD has a single spiral track of data circling from the inside of the disk to the outside. The track is approximately 0.5 microns wide, with 1.6 microns separating one track from the next. The track consists of a series of bumps, 0.5 microns wide, a minimum of 0.97 microns long and 125 nanometres high.
You will often read about "pits " on a CD instead of bumps. They are pits on the aluminium side, but on the side the laser reads from they are bumps. If you could somehow lift the data track off a CD and stretch it out into a straight line, it would be almost 7.4 kilometres.
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