The change in speed of light when it travels through different mediums is called refraction.

While the term "refraction" describes the bending of light as it passes from one medium to another, this bending is caused by the change in speed.

Here's why:

* Light travels at its fastest speed in a vacuum.

* When light enters a denser medium (like water or glass), it interacts with the atoms in that medium, slowing it down.

* This change in speed causes the light to bend, or refract.

So, while the term "refraction" refers to the bending of light, the underlying cause of this bending is the change in speed.