Reading a CD using a laser beam
The following set of diagrams
explains how a laser beam is used to 'read' a CD. The digital output, zeros and ones, results
from the interference at the surface of the aluminium layer.
In the condition shown in the large diagram (Figure 1) part
of the laser beam falls on a plateau and part on a pit. The pit is one quarter of a wavelength
deep and so destructive interference results giving a zero. The following set of small
diagrams (Figure 2) shows the change from zero to one in the reflected intensity.
Laser beam reflected from a
plateau – constructive interference results and so the output signal is high – a one.
Laser beam reflected from the edge of a plateau – destructive
interference results and so the output signal is low – a zero.
Laser
beam reflected from a pit – constructive interference results and so the output signal is high –
a one.
Laser beam reflected from the edge of a plateau – destructive
interference results and so the output signal is low – a zero.
Laser
beam reflected from a plateau – constructive interference results and so the output signal is
high – a one.
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