Question

Why is there no deviation in the path of light when a ray of light strikes a glass slab at the normal incidence?

Answer 1

When a ray of light falls normally to the surface of a glass slab it means it makes an angle of 0^o with respect to its normal. So,

angle of incidence i = 0

and then applying the refraction rules angle of refraction r will also be 0.Thus, no refraction takes place and the ray of light emerges through the medium undeviated.