Explain the refraction of a plane wave by a thin prism.

(N/A) The figure shows a parallel beam incident on a prism at an instant. Its corresponding incident plane wavefront $A_{1}B_{1}$ is normal to the rays. The emergent beam is represented by $A_{2}B_{2}$.
The path length of the ray passing through the base of the prism (from $A_{1}$ to $A_{2}$) is greater than the path length of the ray passing through the apex of the prism (from $B_{1}'$ to $B_{2}'$).
Since the velocity of light in the prism is less than the velocity of light in air,the light takes a longer time to travel the path $A_{1}A_{2}$ compared to the path $B_{1}'B_{2}'$.
As a result,the wavefront at $A_{2}$ lags behind the wavefront at $B_{2}$. Consequently,the emergent wavefront is tilted relative to the incident wavefront,which explains the deviation of light by the prism.