Light incident normally on a plane mirror attached to a galvanometer coil retraces backwards a shown in Fig. A current in the coil produces a deflection of `3.5^(@)` in the mirror. What is the displacement of the reflected spot of light on a screen placed `1.5 m` away ?

Explanation

Angle of deflection, `theta=3.5^(@)` Distance of the screen from the mirror, D=1.5m
The reflected rays get deflected by an amount twice the angle of deflection i.e. 20= `7.0^(@)`.
the displacement d) of the reflected spot of ligh on the screen is given as:
`tan2theta=d/1.5`
`therefore d=1.5 xx tan7^(@)` = 0.184 m =18.4 cm
Hence, the displacement of the reflected spot of light is 18.4 cm.