Why the light ray bend towards the normal when it enters from rarer to denser medium and vice versa?
1 Answer
Hope it helps!
Explanation:
Well, you could use a complete and exact mathematical description using Maxwell Equations BUT this is quite steep and probably not really easy to describe here.
I always use a simpler but perfectly understandable (and reasonable) description using Huygens Principle.
This principle is a kind of geometrical/practical way of considering waves and their propagation:
every point on a wavefront (the crest of the wave) is source of spherical waves whose envelope forms the next wavefront.
Now, when your wave meets the interface with another medium the speed of the wave changes....this produces a "variation" in radius of the resulting spherical waves and so causes a deformation in the next wavefront!
the direction of propagation (perpendiculat to the new wavefront) is altered as well (red pointer arrow)!
As an example, consider a platoon of soldiers marching in formation on, say, tarmac; at a certain point they meet a different surface, say, sand. The speed of the soldiers will change in entering the sandy surface. Entering at an angle, the first soldier of the first line will slow down...then the second....then the third....and so on for all the lines of soldiers changing the direction of the entire platoon!!!!
