critical angle

This term is a technical specification used primarily in optics and physics. It describes a precise threshold of incidence; if the angle of light is smaller than this value, the light escapes the medium, but if it is larger, the light is trapped.

This phenomenon is the foundational principle behind the operation of fiber optic cables and the shimmering effect of mirages. As a specialized scientific term, it is almost exclusively used in academic, engineering, or technical contexts and does not have a metaphorical or casual application in general English conversation.