What occurs in a medium when light travels through it at a different speed than in a vacuum?

When light travels through a medium at a different speed than in a vacuum, refraction occurs. Refraction is the bending of light as it passes from one medium to another, caused by a change in speed. The speed of light is fastest in a vacuum, and when light enters a denser medium, it slows down, leading to a change in direction. This bending can be quantitatively described by Snell's Law.

The phenomenon of refraction is important in various practical applications, such as the design of lenses and optical instruments. For example, when light passes from air into water, it bends towards the normal line, causing objects submerged in water to appear closer to the surface than they actually are.

In contrast, reflection involves the bouncing back of light from a surface, absorption refers to the transfer of light energy into the medium, and interference is a phenomenon that occurs when light waves overlap and combine, which is not directly related to changes in speed as light travels through different mediums. Understanding these concepts is crucial in the study of optics and the behavior of light.