The Fig. shows a device used to measure small twist in a thread. A plane mirror is suspended from a twist free thread. A light ray striking the mirror is reflected on to a screen placed at a distance `D = 1 m` from the mirror.
As the thread is twisted by an angle `theta` (so that mirror rotates by `theta`), the light spot on the screen moves from A to B such that `AB = 0.5 cm`. Find `theta`.
