Two plane mirrors M1 and M2 , placed at right angles, form two sides of a container. Mirror M1 is inclined at an angle θ0 to the horizontal. A light ray AB is incident normally on M1 . Now the container is filled with a liquid of refractive index μ so that the ray AB is first refracted, then reflected at M1 . The ray is next reflected at M2 and then comes out of the liquid surface making an angle θ with the normal to the liquid surface. Find θ .