In the diagram, Media A, B, C, and D each represent different mediums. The phenomenon depicted in the diagram is BEST described as refraction.
Refraction is the bending of light as it passes from one medium to another with a different optical density. In this case, the change in the path of the light as it transitions from one medium to another is evident.
When light travels from a medium of lower optical density to a medium of higher optical density, it bends towards the normal line whereas when it goes from a medium of higher optical density to a medium of lower optical density, it bends away from the normal line.
This bending of light is a fundamental characteristic of refraction and is responsible for various optical phenomena we observe in our daily lives, such as the bending of a straw in a glass of water.