Question

How does a change in speed cause a wave to change direction​

Answer 1

Refraction is an effect that occurs when a light wave, incident at an angle away from the normal, passes a boundary from one medium into another in which there is a change in velocity of the light. ... The wavelength decreases as the light enters the medium and the light wave changes direction.

Answer 2

When a wave enters a new medium at an angle, one side of the wave changes speed before the other side. This causes waves to bend. Bending occurs because the different parts of the wave travel at different speeds.