why does a ray of light not change direction when it passes from from one medium ti another at normal incidence(perpendicular to the boundary)
Answers
Answer:
Explanation:
For a ray of light coming in at normal incidence, every direction that it could turn is equivalent to every other. On what physical basis could it possibly "choose"? Imagine that multiple people are watching the ray of light hit the interface, but that they're all watching from different angles. If the light ray deviated, some of the people would see it deviate to the right, others to the left, others at other angles. However, they would all see the situation before the ray hit the interface as being totally the same (locally, at least). So, if there were a deviation, it would be physically impossible for us to predict what direction it would be in!
There are also more-rigorous arguments, involving conservation laws, the continuity of E&M fields at boundaries, geometrical derivations of Snell's law (of which this is a special case), and so on. However, I like the symmetry argument above: if no direction is any different from any other, there's no way for it to "choose", and so it can't deviate at all!