CD-ROM

It is an optical  ROM.Prerecorded data can be read out.The manufactures writes data on CD-ROMs. The disk is made up of a resin.such as poly carbonate. It is coated with a material which will change when a high intensity laser beam is focused on it.The coating material is highly reflective,usually aluminium.The high intensity laser beam forms a tiny pit along a track to represent 1. For reading the data laser beam of less intensity is employed. A laser systems needs 25mW for writing whereas only 5 mW for reading.In some cases separated laser beams are employed:one for writing and the other for reading.the reflected laser is sensed by photodiode to read data.The intensity of the reflected light of laser changes as it encounter pit. A pit spreads the light so that the photodiode receives little reflected light.But surface without pit reflect sufficient light to the photodiode. Thus the change in reflected light is sensed and converted into electrical signals for data reading purpose.

If the coating of the special material on the optical disk is thin film,a hole is formed when the laser beam falls on it.If the layer is a thick film a pit is formed.In some process of thick film,a bubble is formed instead of a pit.The unaltered areas between the pits are known as lands. 

First of all,a master disk is prepared.By molding a special plastic to the master disk.CD-ROMs are produced on mass scale.The plastic copy of the data is then covered with a clear plastic layer and backed with a reflecting coating.