Light incident normally on a plane mirror attached to a galvanometer coil retraces backwards a shown in Fig. 6(a). 14. A current in the coil produces a deflection of `3.5^(@)` in the mirror. What is the displacement of the reflected spot of light on a screen placed `1.5 m` away ?
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Here, `theta = 3.5^(@), x = 1.5 m, d = ?` when the mirror turns through an angle `theta`, the reflected ray turns through double the angle, `2 theta = 2 xx 3.5^(@) = 7^(@) = (7 pi)/(180)rad` From Fig. `tan 2theta = (SA)/(OS) = (d)/(1.5)` `d = 1.5 tan 2 theta = 1.5 tan 2 theta ~~ 1.5 (2 theta) = 1.5 xx (7pi)/(180)m = 0.18m`
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