A beam of light from a source $L$ is incident normally on a plane mirror fixed at a certain distance

Question

A beam of light from a source $L$ is incident normally on a plane mirror fixed at a certain distance $x$ from the source. The beam is reflected back as a spot on a scale placed just above the source $L$. When the mirror is rotated through a small angle $	heta$, the spot of the light is found to move through a distance $y$ on the scale. The angle $	heta$ is given by\

Professor PC Thomas Classes · Answer

(d) : When mirror is rotated by $	heta$ angle reflected ray will be rotated by $2 	heta$. For small angle $	heta$,\ $	an 2 	heta \approx 2 	heta=\frac{y}{x}$\ $	herefore 	heta=\frac{y}{2 x}$\