Why does light have to hit the border between two media at an angle for refraction to occur?

Answer 1

This could be easily understood with the idea of plane wavefront. Huygens have given the concept of wavefront. That has been defined as the locus of all points in the medium which get vibrated simultaneously and are vibrating in phase. More over ray concept is old one. Any ray of light will be normal to the new concept ie wavefront.

So as ray hits normally on the surface, then all points on the wavefront hit simultaneously on the surface of separation and secondary wavelets would be produced at the same instant. So the wavefront would continue as it is. Hence ray goes un deviated. On the other hand if the ray hits at an inclined position with the surface of separation, then the wavefront would be falling in an inclined way. So those reach earlier the surface would start giving rise to secondary wavelets which would traverse at less speed in the other media. Thus the successive points giving rise to secondary wavelets would make the wavefront to be inclined at a different angle with the surface of separation. Hence the ray gets refracted.

We can imagine as if a military parade is crossing the border right from cemented road to a region full of sand. On cemented road one can walk easily so at high speed. But in sand speed would be reduced. So the parade as a whole would change the direction of marching forward.

Answer 2

When light passes through a medium at an angle, its speed changes unevenly, causing the light rays to bend. This bending of light is known as refraction. If light hits the border perpendicularly, the speed change would be uniform and no bending would occur.