1. what is principle of reversibility of light? 2. when light passes through a rectangular glass slab, the emergent ray is parallel to incident ray(produced).why?

By the principle of reversibility of light, the light makes the same angle, i_AIR, with the normal in air as it did originally when travelling in the opposite direction. The principle of reversibility states that light will follow exactly the same path if its direction of travel is reversed.

 when light passes from air into glass and then back into air, if the refracting surfaces are not parallel (like a slab), then the emerging ray takes a different path than the initial incident ray. They remain parallel. The angle between the emerging ray and the incident ray is called the angle of deviation.