1. The problem statement, all variables and given/known data Water level in the tank is decreasing at a constant rate of 1 cm/s. A small metal sphere is moving downwards with a constant velocity of 5cm/s. Base of the tank is a concave mirror of radius 40 cm. Find the velocity of the image seen (refractive index of water=4/3) a)directly b)after reflection at the mirror (Answer: a)4 cm/s downwards (b) 59/4 cm/s upwards) 2. Relevant equations 3. The attempt at a solution Honestly I don't know where to start with for (a). The water level isn't stationary here. I tried something random but that gives an incorrect result. Let x be the height of water level above the mirror and y the height of sphere from mirror. The real depth of sphere is (x-y). The apparent depth is ##3/4(x-y)##, differentiating with respect to t, ##3/4(dx/dt-dy/dt)=3/4\times 4=3 cm/s## which is wrong. Any help is appreciated. Thanks!