Question

explain why there is no refraction
with normal incidence​

Answer 1

Answer:

This is one of the ways to think about refraction: it occurs to conserve the in-plane wave vector. When light is at normal incidence, the in-plane wave vector is zero, so there's no need for refraction.

Answer 2

Answer:

In such case, the incident ray goes along the normal to the surface. ... In other words,the ray which is incident normally on the interface between the two different media, propagates un deviated from one medium to other and there is no refraction.