CD-ROM
CD-ROM is
an optical disk and contains data that can only be read. We can’t write or
erase data from CD-ROM. CD-ROM stands for Compact Disk-Read Only Memory. CD-ROM
is very popular in the mid of 1990s and 2000s. A CD-ROM has three layers.
CD-ROM has transparent substrate on its base. On the base, there is a
reflecting layer and this reflecting layer stores data. On the reflecting
layer, there is a protective coating.
The
information to be stored is digitized, that is, the data can be represented by
binary numbers, which are made up of '1' and '0' digits. This information is
stored on the metal layer. The data is represented using pits (1) and space
between pits is called land (0).
Laser beam
is used for storing data on CD-ROM. Data is stored on a single spiral track.
Each track is divided into sector. The track has length of 5 km and 650 mb of
data can be stored. The tracks have high density with a gap of 1.6 microns.
Laser beam is
used to read the data from compact disk. CD is moving on a head at very high
rpm. A laser is shone onto the reflective surface of the disc to read the
pattern of pits (1) and lands (0). The depth of the pits is approximately
one-quarter to one-sixth of the wavelength of the laser light used to read the
disc. The laser rays reflected by the pits and the lands have a path difference
of half the wavelength. It leads to destructive interference, means, the two
rays cancel each other and no reflected ray is resulted, and this produces a
'1' digit. The path difference is zero for two rays both reflected by the pits,
or both by the lands. No destructive interference occurs and we say it is a '0'
digit. Therefore we have digit '1' at the edge of pits and digit '0' elsewhere.
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