Interface
The question is too vague for a definitive answer; depending on context, "surface", "interface", or even just "boundary" might be used.
IF the refractive indexes of the two materials are not the same number, and IF the light is not traveling perpendicular to the boundary between them, then the direction of the light will change at the boundary. This happens because of the slight difference in the speed of light in the two materials. The process is called "refraction" of light. Without it, eyes and other lenses would not work.
Light refracts when is crosses the boundary between two materials. That means that is the light is coming in at one angle, then it will change direction at the interface and have a different angle moving out from the other side. There is an old formula called Snell's Law that predicts the exit angle in terms of the properties of the material, i.e. in terms of the index of refraction. There is reflection as well. Some light bounces off of a boundary and some is transmitted.
The direction of light propagation can be changed at the boundary of two media having different densities. This property is called refraction, and is illustrated in the following figure for the boundary between air and water.
refractionRefraction
Boundary or interface.
The boundary between the troposphere and the stratosphere is called the tropopause.
The boundary between the troposphere and the stratosphere is called the tropopause.
Such a boundary is called a front.
The question is too vague for a definitive answer; depending on context, "surface", "interface", or even just "boundary" might be used.
Convergent boundary's.
The boundary between the mantle and the core is called the core-mantle boundary and also The Gutenberg Discontinuity which marks the upper boundary of the D'' (D Double Prime) layer.
The boundary of mantle and core are called Gutenberg Discontinuity
A boundary where the plates are moving apart is called a divergent boundary. It is also called a constructive boundary because new crust is made there.
meterology
A front
It is called a fault