Light incident normally on a plane mirror attached to a galvanometer coil retraces backwards as shown in figure 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?

Solution

We know that if the mirror is turned by an angle θ, the reflected ray turns by an angle 2θ.

The current in the coil has produced a deflection of 3.5° in the mirror, therefore, the reflected ray will be deflected by an angle 2 x 3.5° = 7°.

From the figure, we have

\frac{d}{1.5} = \tan 7 °

[\tan θ = \frac{perpendicular}{base}]

\Rightarrow d = 1.5 \tan 7 ° = 1.5 \times 0.123 = 0.184 m = 18.4 cm .

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Wave Optics

Light incident normally on a plane mirror attached to a galvanometer coil retraces backwards as shown in figure 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?

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