There is no physical contact between the surface of the CD and the reading device.
The diameter of the laser beam is so small that storage tracks can be written very close together, allowing more data to be stored in a smaller space.
Hard Disk Drives vs. CD-ROMs
With the cost of hard disk drives falling and the amount of available data storage rising, the hard drive is still king of the storage media. Optical data-storage devices hold their place as removable media and as the media of choice for archival data storage.
A CD platter is composed of a reflective layer of aluminum applied to a synthetic base that is composed of polymers. A layer of transparent polycarbonate covers the aluminum. A protective coating of lacquer is applied to the surface to protect it from dust, dirt, and scratches.
CD-recordable (CD-R) discs use materials other than aluminum. They often have a yellow or green cast on the data side. Not all CD-ROM readers are able to read these discs-some older readers based on IDE (Integrated Drive Electronics) are incompatible with CD-R technology.
Data is written by creating pits and lands on the CD's surface. A pit is a depression on the surface, and a land is the height of the original surface. The transition from a land to a pit, or a pit to a land, represents a binary character of 1. Lands and pits represent binary 0. The reading of data is based on timing-the speed at which the CD is rotating-and the reflection of light. If no data is on the disk, the reflectivity will not change and the CD will read a series of binary 0s. There are approximately 4 to 5 million pits per CD. They are arranged in a single outward-running spiral (track) approximately 3.75 miles (6 kilometers) long. The distance between each element is 1.6 thousandths of a millimeter.