The boundary between two materials relating to light is called an interface.

Here's why:

* Interface: This term describes the common surface where two different substances meet.

* Light and Interfaces: When light travels from one medium to another (like from air to water), it encounters this interface. This interface causes the light to change direction (refraction) or even be reflected back (reflection).

So, the interface is the key point where light's behavior is influenced by the change in materials.