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Total Internal Reflection & its Applications

Total Internal Reflection & its Applications


  • Total internal reflection is an optical phenomenon that happens when a ray of light strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface.
  • The critical angle is the angle of incidence above which the total internal reflection occurs.
  • When light crosses a boundary between materials with different kinds of refractive indices, the light beam will be partially refracted at the boundary surface, and partially reflected.
  • However, if the angle of incidence is greater (i.e. the ray is closer to being parallel to the boundary) than the critical angle — the angle of incidence at which light is refracted such that it travels along the boundary — then the light will stop crossing the boundary altogether and instead be totally reflected back internally.
  • This can only occur where light travels from a medium with a higher refractive index to one with a lower refractive index.
  • For example, it will occur when passing from glass to air, but not when passing from air to glass.


  • This effect is used in optical fibres to confine light in the core, which are used in endoscopes and telecommunications.
  • Rain sensors use the phenomenon of total internal reflection to control automatic windscreen/windshield wipers.
  • Another interesting application of total internal reflection is the spatial filtering of light.
  • Prismatic binoculars use the principle of total internal reflections to get a very clear image
  • Some multi-touch screens use frustrated total internal reflection in combination with a camera and appropriate software to pick up multiple targets.
  • Gait analysis instrument, CatWalk uses total internal reflection in combination with a high speed camera to capture and analyze footprints of laboratory rodents.
  • Fingerprinting devices, which use frustrated total internal reflection in order to record an image of a person’s fingerprint without the use of ink.

Besides that total internal reflection is useful in everyday life too. Total internal reflection can be observed while swimming, when one opens one’s eyes just under the water’s surface. If the water is calm, its surface appears mirror-like.

  • One can demonstrate total internal reflection by filling a sink or bath with water, taking a glass tumbler, and placing it upside-down over the plug hole (with the tumbler completely filled with water).
  • While water remains both in the upturned tumbler and in the sink surrounding it, the plug hole and plug are visible since the angle of refraction between glass and water is not greater than the critical angle.
  • If the drain is opened and the tumbler is kept in position over the hole, the water in the tumbler drains out leaving the glass filled with air, and this then acts as the plug. Viewing this from above, the tumbler now appears mirrored because light reflects off the air/glass interface.
  • Another common example of total internal reflection is a critically cut diamond. This is what gives it maximum sparkle.




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