Physics - Geometrical Optics Question with Solution | TestHub

The initial deviation by the mirror is $180^{\circ }-2\times 45^{\circ }=90^{\circ }$.

For a prism with a small apex angle $\alpha$ and refractive index $n$,

the deviation is $\delta =(n-1)\alpha$.

Here, $\delta =(1.50-1)\times 4^{\circ }=0.5\times 4^{\circ }=2^{\circ }$.

The total initial deviation is $90^{\circ }+2^{\circ }=92^{\circ }$.

If the total deviation is to be $90^{\circ }$, the mirror must be rotated by $1^{\circ }$ anticlockwise.