A simple experiment can tell the difference between a CD and a higher-data-densi

Question

A simple experiment can tell the difference between a CD and a higher-data-density DVD. The tracks of pits on an optical data disc can act like a grating when laser light is reflected from its surface. The figure below shows a simple experimental configuration to produce an interference pattern from a data disc using a red laser pointer ( = 650 nm). In this particular experiment, the point on the disc struck by the laser is positioned 14 cm above the tabletop. When the disc is oriented at a 45° angle with respect to the vertical, the reflected laser light shows an interference pattern where the first maximum is 6.3 cm beyond the reflected beam (directly below the disc).(a) What is the spacing of the tracks on the CD?

Explanation / Answer

The first bright spot (m = 1) is 6.3 cm from the reflected ray (which
corresponds to the central maximum), and that the table is 14 cm from the disc, so we
can find the angle for the first ray:

tan theta = 6.3/14

theta = 24.22

Now, dCD = m*Wavelength/sin Theta = 650*10^-9/ Sin 24.2

spacing of the tracks on the CD = 1.6*10^-6 m

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