The angle between two flat mirrors is changed by rotating one of the mirrors around the edge of another with a constant angular velocity $ω = 1.5 deg / sec$ . Source of light S is located as shown in Figure, the distance $h = 10 cm$ . At the initial moment mirrors were in the same plane ( $ϕ = 18 0^{\circ}$ ).

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The inclined mirror rotates with ω = 1.5°/s; the image in it rotates with angular speed 2ω.

(A) When the number of images becomes three, $4 5^{\circ} = ω t = 1.5 t \Rightarrow t = 30 sec$ Correct.

(B) After 40 s the mirror has turned 60°; the source-image distance is $2 h sin 3 0^{\circ} = 2 \times 10 \times \frac{1}{2} = 10 cm$ Correct.

(C), (D) Speed of the image at the initial moment: $v = 2 ωh = 2 \times \frac{1.5 π}{180} \times 10 = \frac{π}{6} cm / s$ so (D) is correct and (C) is wrong.

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