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Solution: Click For PDF Version Let
be the frequency of incident light. According to light quantum
hypothesis, each photon carries energy
 ,
where h is the Planck
constant.The object on which circularly polarized light is incident is a flat disk of diameter 5.20 mm. The power of the beam is 106 W. Therefore, the number of photons that are incident on the disk per second will be
We will assume that all incident photons are absorbed by the disk. We will use the quantum property that each photon carries angular momentum of magnitude
Therefore, the total amount of angular momentum transferred to the disk per second by the photons will be
Rotational
inertia of a circular disk of radius R
and mass M is
Therefore,
the time in which the disk will attain rotational speed of
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(we may assume that the disk is free to
rotate about its axis). Let
the wavelength of the incident light be 516
nm.
.
.
.
When the disk attains angular speed of
