A transparent cube with index of refraction n = 1.6 and side length a = 4.0 cm is free to

Question

rotate about an axis passing through its center. A laser beam is aimed at the cube. After passing through the cube the beam hits a screen on the other side forming a spot. The spot is displaced a distance y from the straight line path. The cube is then rotated at a constant angular velocity ω= 10 rad/s. As the cube rotates the spot moves. What is the speed of the spot at the instant that y = 0?