Critical angle and Incidence angle

Critical angle noun - The smallest angle of incidence for which light is totally reflected.

Incidence angle and critical angle are semantically related. In some cases you can use "Incidence angle" instead a noun phrase "Critical angle".

Incidence angle noun - The angle that a line makes with a line perpendicular to the surface at the point of incidence.

Critical angle and incidence angle are semantically related. Sometimes you can replace phrase "Critical angle" with "Incidence angle", this phrases are similar.