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Ray Optics and Optical Instruments — Physics STD 12 Science — Question

Bihar BoardEnglish MediumSTD 12 SciencePhysicsRay Optics and Optical Instruments2 Marks
Question
Light incident normally on a plane mirror attached to a galvanometer coil retraces backwards. 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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Answer

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 = 7.0°
The displacement (d) of the reflected spot of light on the screen is given as:
$\tan2\theta=\frac{\text{d}}{1.5}$
$\therefore \ \text{d}=1.5\times\tan7^\circ=0.814 \ \text{m}=18.4 \ \text{cm}$
Hence, the displacement of the reflected spot of light is 18.4 cm.

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