The specific angle of incidence at a boundary between two media where refracted light travels exactly along the interface. Any light hitting the boundary at a steeper angle than this will bounce back entirely into the denser medium. It dictates how optical fibers trap light and how prisms steer beams.
Make sure the beam hits the prism face past the critical angle or we'll lose half our power to transmission.
In fiber optics, the critical angle determines the numerical aperture, dictating how much light the fiber can actually collect.
Related terms
Total internal reflection
The complete reflection of a light ray at the boundary of a medium with a lower refractive index.
Snell's law
The fundamental formula calculating how much a light ray bends when it crosses the boundary between two different transparent materials.
Evanescent
Describing an electromagnetic wave that decays exponentially rather than propagating.