Question

A small plane mirror kept at the centre of a sphere of diameter $3m$, makes $12$ revolution per second. A thin light beam is made incident on the mirror. The linear speed of the light spot on the sphere, formed after reflection from the mirror is :

• A. $18\pi m/s$
• B. $36\pi m/s$
• C. $72\pi m/s$
• D. $144\pi m/s$

Answer 1

The correct option is D $144\pi m/s$

When plane mirror rotates through an angle $\theta$ , the reflected ray rotates through an angle 2$\theta$. So spot on the screen will make 2 x 12 = 24 revolution per second.

To find the velocity

v=2$\pi$R x frequency

=2$\pi$ x 3 x 24

=144$\pi$ m/s

D is the Correct answer.