Question

what is the cause of refraction?

Answer 1

The Cause of Refraction. We have learned that refraction occurs as light passes across the boundary between two media. Refraction is merely one of several possible boundary behaviors by which a light wave could behave when it encounters a new medium or an obstacle in its path.

Answer 2

Refraction occurs when a wave moves from a material with one density into a medium with a different density.  All waves refract, however we most frequently use the properties of refraction with electromagnetic waves, especially light.

Refraction means that the light waves  are bent, or change direction when they enter a new medium.