# Q 9.31 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 3.5o of the mirror. What is the displacement of the reflected spot of light on a screen placed 1.5 m away?

P Pankaj Sanodiya

Given

Angle of deflection $\delta = 3.5^0$

The distance of the screen from the mirror $D=1.5m$

The reflected rays will bet deflected by twice angle of deviation that is

$2\delta =3.5*2=7^0$

Now from the figure, it can be seen that

$tan2\delta =\frac{d}{1.5}$

$d=1.5*\tan2\delta = 2*\tan7=0.184m=18.4cm$

Hence displacement of the reflected spot of the light is $18.4cm$.

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