Question

A small angle prism has a prism angle $|A|=4^{\circ}$ and refractive index $=1.5$.It is placed with its base horizontal in front of a vertical mirror. A horizontal ray of light passes through the prism and is reflected back from the mirror. By what angle(in degree) the mirror should be rotated so that the reflected ray becomes horizontal ?

Answer 1

Deviation produced by prism
$=(\mu-1) A $
$=(1.5-1)\left(4^{\circ}\right)$
$\delta=2^{\circ}$
So ray is incident on mirror at an angle $2^{\circ}$ to horizontal.
For making reflected ray horizontal it has to turn clockwise by an angle of $2^{\circ}$.
Hence mirror should be rotated by an angle $1^{\circ}$ in anticlockwise direction.