Refraction when angle of incidence is equal to 0 degree

Question

Ok i know that mathematically or by snell 's law it can be proved but i want to know that what is its physical significance? I mean why it does not refract at 0 degree?

Answer 1

Looked at using the Huygens-Fresnel principle: each point in the wavefront will arrive on the surface at the same time. Each point is a new source emitting with the new wavelength, but as they all arrived at once, their waves are all in step so the wavefront stays the same.