Total internal reflection is an optical phenomenon in which a light ray is completely reflected back into a medium when it strikes the boundary with a second medium, without any refraction into the second medium.
This occurs only under two conditions:
- The light is traveling from a medium with a higher refractive index to one with a lower refractive index (e.g., from water to air or glass to air).
- The angle of incidence is greater than the critical angle for the pair of media. The critical angle is the angle of incidence at which the refracted ray would travel exactly along the boundary.
When these conditions are met, instead of bending into the second medium, the light bounces entirely back into the original medium, like a mirror reflection.
Total internal reflection is responsible for effects such as the sparkle in diamonds, the functioning of optical fibers, and mirages. It allows light to be efficiently contained and guided within transparent materials.