Light incident normally on a plane mirror attached to a galvanometer coil retraces backwards as shown in Fig. 9.33. A current in the coil produces a deflection of `3.5^(@)` of the mirror. What is the displacement of the reflected spot of light on a screen placed 1.5 m away?

Explanation

`rArr` When mirror is rotated by `theta` and its position changes from M to M. , then reflected ray will be rotated by `2 theta` . Hence, reflected ray will be obtained at Q from P on screen.
`therefore angle POQ = 2 theta`
`= 2 xx 3.5^@`
` = 7^@`

Now, from figure,
`tan 2 theta = d/D`
`= d/1.5`
`rArr` Displacement of relected point on screen,
`d = D tan 2 theta`
`therefore d= 1.5 xx tan 7^@`
`= 1.5 xx 0.1228`
`0.1842 m `
`= 18.42 cm `