The critical angle for light approaching the boundary between two different media is the angle of incidence above which it undergoes total internal reflection. The critical angle is measured with respect to the normal at the boundary between two media.
- Internal reflection is a common phenomenon so far as visible light is concerned but occurs with all types of electromagnetic radiation.
- Internal reflection takes place when light travelling through a medium:
- For example, internal reflection takes place when light reaches air from glass and at an angle greater than the critical angle.
- In normal conditions, light is partially refracted and partially reflected because of irregularities in the surface at the boundary.
- It is only when the angle of incidence is greater than the critical angle for all points along a boundary that total internal reflection takes place.