CD-ROMs (Compact Disk – Read Only Memory) are compact disks which are used for storage. A user cannot write data to a CD-ROM. It is read only. Here’s how a CD-ROM is made: Holes of diameter 0.8 micrometer are made in a coated glass disk by using an infrared laser. This glass disk is used to make a mold and molten poly-carbonate resin is injected in to the holes. After that a thin layer of reflective aluminum is deposited on the poly-carbonate. It is then topped by a protective lacquer and at the top a label is put. Holes in poly-carbonate substrate are called pits and un-burned areas in between the pits are known as lands. Then pits and lands are written in a single continuous spiral arrangement. When the CD-ROM is played a laser diode is used for passing infrared light having a wavelength of 0.78 microns on the pits and lands. When the light reflects from the pit its wavelength is half the wavelength of the land. Now to, play the CD-ROM it is rotated at a constant linear velocity.