Total internal reflection

Total internal reflection occurs when incoming light traveling through a medium strikes the boundary with a second medium at angles greater than a certain critical angle with the result that no light crosses the boundary and so all the light is reflected back into the medium.

  • Internal refraction only takes place if the refractive index is lower for the medium on the other side of the boundary.
  • The critical angle is the angle of incidence above which internal reflection occurs.
  • The critical angle is measured with respect to the normal.
  • The normal is an imaginary line drawn in a ray diagram perpendicular to, so at a right angle to (900), to the boundary between two media.

https://en.wikipedia.org/wiki/Total_internal_reflection