1. The problem statement, all variables and given/known data A small mirror of area A and mass m is suspended in a vertical plane by a weightless string. A beam of light of intensity I falls normally on the mirror and the string is deflected from the vertical by a very small angle A. Assuming the mirror to be perfectly reflecting, to obtain an expression for A. Ans: 2IA/(mgc), where c is the speed of light. 2. Relevant equations Energy should be conserved in the entire system. If the mirror moves up by an angle it has gained potential energy, and the only way which it could have got this is from light. But E=Hf (f is the frequency of light) so the light's frequency decreases upon complete reflection. What am I missing ? 3. The attempt at a solution None, I do not understand why the mirror moves in the first place. I do not understand what am I missing from Conservation of energy. Or is the question totally incorrect. Thank you for all the possible help.