Critical angle

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 strikes the boundary of another medium with a lower refractive index at an angle greater than the critical angle.
  • For example, internal reflection takes place when light reaches air from glass at an angle greater than the critical angle, but not when light reaches glass from air.
  • In general, light will be partially refracted and partially reflected because of irregularities in the surface at the boundary.
  • However, if the angle of incidence is greater than the critical angle for all points at which light strikes the boundary then no light will cross the boundary, but will instead undergo total internal reflection.