In general, when a ray of light pasess through a lens, there occurs a change in its direction of propagation . Why ?

Explanation

The working of a lens is similar to that of a triangular prism. When a ray of light passes through a lens, it is refracted twice : When entering the lens and when emerging from the lens . There is a change in its direction of propagation everytime and as both the changes occurs in the same sense, the direction of propagatio of the emergent ray is different from that of the incident ray.