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A ray of light is reflected by two plane mirror mutually at right angles to each other. What is the deviation of reflected ray from the incident ray

$(a)\;90^{\circ} \\ (b)\;60^{\circ} \\ (c)\;180^{\circ} \\ (d)\;0^{\circ}$

Normal $N_1$ perpendicular to normal $N_2$
$\angle ABC + \angle BCD =2 \theta +2 (90 -\theta)$
$\qquad=180^{\circ}$
Rays after reflection from two mutually perpendicular mirrors becomes anti parallel.
Hence c is the correct answer.