Optical disk storage and cd - ROM:

Optical disk storage is relatively new alternative to magnetic storage. Digital data is burnt into the surface of a plastic disk coated with a low wetting point metal by a laser beam. The presence or absence of a hole corresponds to one bit and can be read by another laser: This technology offers a high capacity storage at low cost, and in a form allowing rapid access (analogous to magnetic disks). 

WORM (Write Once Read Many Times) disks are ideal for archival purposes allowing organisation to store large volumes of documentation on one disk. For instance a 12 inch disk can hold about 2.5 GB at a cost of few hundred rupees. Storage capacities of 5.0 GB is now available. But, the major problem with WORM technology is the absence of any industry standards. Now 10 inch disks (compact version CD-WORM) is being developed. 

The CD-ROM (Compact Disk Read Only Memory) is a 4.75" disk capable of holding 650 MB. The CD-ROM has become a means of distributing databases and documentation (including journals and patents) in electronic form.