Question

A small plane mirror is placed at the centre of a spherical screen of radius $R$. A beam of light is falling on the mirror. If the mirror makes $n$ revolutions per second, the speed of light on the screen after reflection from the mirror will be

• A. $4\pi nR$
• B. $2\pi nR$
• C. $nR/2\pi$
• D. $nR/4\pi$

Answer 1

Answer: (A)

$4\pi nR$

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

$W^{\prime }=2w=2\times 2\pi n=4\pi n$

now $V=w^{\prime }R=4\pi nR$