An optical disc is a metal-coated polycarbonate disc covered by protective clear plastic with a 20 kilometer long (or longer) spiral, with pits inscribed along the spiral. Each pit is between 1.3 and 4 micrometers (millionths of a meter) long, so a laser is necessary to focus light on such tiny pits in the disc. A human hair is about 75 micrometers wide; a phonograph groove is about 100 micrometers wide.
On a music disc, the length and frequency of occurrence of the pits matches the sound's pitch and loudness. Unlike a phonograph record, reading speed is high, scratches will not harm it, the disc lasts longer than a human does, and there is no degradation of the reading surface over repeated readings. Human mouths produce sounds that are vibrations in the air, these vibrate from the lowest bass of about 73 hertz (73 cycles a second) to the highest soprano of about 1.5 kilohertz (1,500 cycles per second). Because we can hear only up to about 20 kilohertz, once we sample a sound at twice that speed or higher we capture all that any human can hear.
A compact disc (CD) is just a small optical disc; instead of music it can just as easily store any sequence of pits. For example, digital cameras and scanners can convert any scene into a series of bits, and we can store these bits as pits in an optical disc.
An optical disc can hold from 550 to over 1,000 megabytes (1 gigabyte). So one small light disc can store up to 1,000 textbooks or 2,000 novels. Sony chose the size of compact discs (72 minutes) so that one would contain all 66 minutes of Beethoven's Ninth Symphony; beside convenience, there is no other reason for them to be so small. Further, because they are circular, their area grows as the square of their radius, so a disc of double the width would hold 4 times as much information. Larger discs can hold 5,000 books--a truckload. A few dozen can hold a trainload. A few thousand can hold all 20 million books in the Library of Congress.