Understanding Total Internal Reflection


Experimental verifications of total internal reflection

Experiment 1:
  • Take a glass beaker with clear water in it.
  • Stir the water a few times with a piece of soap, so that it becomes a little turbid.
  • Take a laser pointer and shine its beam through the turbid water.
  • You will find that the path of the beam inside the water shines brightly.
  • Shine the beam from below the beaker such that it strikes at the upper water surface at the other end.
  • It undergoes partial reflection and partial refraction.
  • Now direct the laser beam from one side of the beaker such that it strikes the upper surface of water more obliquely.
  • Adjust the direction of laser beam until you find the angle for which the refraction above the water surface is totally absent.
  • The beam is totally reflected back to water. This is called as total internal reflection.

Experiment 2:
  • Pour the turbid water in a long test tube.
  • Shine the laser light from top.
  • Adjust the direction of the laser beam such that it is totally internally reflected every time it strikes the walls of the tube.
  • This demonstrates total internal reflection.


Total internal reflection (TIR)

When light travels from an optically denser medium to a rarer medium at the interface, it is partly reflected back into the same medium and partly refracted to the second medium. This reflection is called the internal reflection.


Total internal reflection is defined as the complete reflection of a light ray at the boundary of two media, when the ray is in the medium with greater refractive index.