When light passes from one medium to another. Depending on the density, it would experience a change in velocity. Why would its vector change as well?

Question

For example, when light passes through water its velocity decreases and refraction occurs. Why? Why is there a change in direction. If I slow down a car I don’t suddenly turn left.

Answer 1

For example, when light passes through water its velocity decreases and refraction occurs. Why? Why is there a change in direction. If I slow down a car I don’t suddenly turn left.

Imagine your car is sliding down an icy hill at a 45° angle, just ahead the ice ends at a 0° angle. One side will hit the pavement before the other. The side that hits first will slow down and the car will pivot while the end on ice continues at its original speed. When the car is fully on the pavement it will have refracted to a new angle. The slower medium is the pavement with a higher coefficient of friction than the ice.

Since most people drive their cars in a 90° angle (and not 45°), there is no rotation since the motion is perpendicular to ice/pavement interface.

A wavefront ( light, sound) approaching at a 45° will undergo the same pivot as one end slows down while the other end remains at the same speed until the wavefront is completely in the slower medium. For light the slower medium has a higher index of refraction.