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Question
Light incident normally on a plane mirror attached to a galvanometer coil retraces backward 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
Angle of deflection, θ = 3.5°
Distance of the screen from the mirror, D = 1.5 m
The reflected rays get deflected by an amount twice the angle of deflection, i.e., 2θ = 2 × 3.5° = 7°.
The displacement (d) of the reflected spot of light on the screen is given as:
tan 2θ =
∴ d = 1.5 × tan 7° = 0.184 m = 18.4 cm
Hence, the displacement of the reflected spot of light is 18.4 cm.
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