Question

In a square room ABCD a small plane mirror M is rotating with $\omega =\pi /60$ rad/sec about a vertical axis through the centre of the room. The source S directs a thin parallel beam of ray at the centre of the mirror. Time taken by reflected spot of light to cross the wall DA is?

• A. $10$sec
• B. $12.5$sec
• C. $15$sec
• D. $20$sec

Answer 1

The correct option is C $15$sec

Angular velocity of rotating plane mirror $=\omega =\frac{\pi }{60}$

So angular velocity of reflected spot light $=\frac{\pi }{30}$ i.e double of $\omega$

Now from D to A there is $\frac{\pi }{2}$ angle,

So for that time required is 15 seconds.i.e,

$\pi \to 30sec$

$\frac{\pi }{2}\to 15sec$