<P> CD data is represented as tiny indentations known as "pits", encoded in a spiral track moulded into the top of the polycarbonate layer . The areas between pits are known as "lands". Each pit is approximately 100 nm deep by 500 nm wide, and varies from 850 nm to 3.5 μm in length . The distance between the tracks, the pitch, is 1.6 μm . </P> <P> A motor within the CD player spins the disc to a scanning velocity of 1.2--1.4 m / s (constant linear velocity)--equivalent to approximately 500 RPM at the inside of the disc, and approximately 200 RPM at the outside edge . (A disc played from beginning to end slows its rotation rate during playback .) </P> <P> The program area is 86.05 cm and the length of the recordable spiral is (86.05 cm / 1.6 μm) = 5.38 km . With a scanning speed of 1.2 m / s, the playing time is 74 minutes, or 650 MiB of data on a CD - ROM . A disc with data packed slightly more densely is tolerated by most players (though some old ones fail). Using a linear velocity of 1.2 m / s and a narrower track pitch of 1.5 μm increases the playing time to 80 minutes, and data capacity to 700 MiB . </P> <P> A CD is read by focusing a 780 nm wavelength (near infrared) semiconductor laser housed within the CD player, through the bottom of the polycarbonate layer . The change in height between pits and lands results in a difference in the way the light is reflected . By measuring the intensity change with a photodiode, the data can be read from the disc . In order to accommodate the spiral pattern of data, the semiconductor laser is placed on a swing arm within the disc tray of any CD player . This swing arm allows the laser to read information from the centre to the edge of a disc, without having to interrupt the spinning of the disc itself . </P>

Based on year 1 sales of compact discs recorded by an artist