Total internal reflection (TIR)

Critical angle

Imagine a ray of light entering the air from a block of glass; it is obvious that it will get refracted as it enters a different medium. If we increase the angle of incidence of the ray, the angle of refraction will increase as well.

Eventually, at one particular angle called as the critical angle, the refracted ray emerges along and parallel to the surface of the block. Most of the light is reflected inside the block.

Now, if the angle of incidence is extended even more, there comes a point where all the light gets reflected within the glass block!

This occurrence is called as Total Internal Reflection (TIR).

The term Total Internal Reflection is used here because:

  1. TOTAL: as 100% of the light is reflected within
  2. INTERNAL: as it occurs within the glass block
  3. REFLECTION: as the ray of light is entirely reflected (and not refracted!)

Note! For Total Internal Reflection to occur, the angle of incidence must be greater than the critical angle.

FUN FACT: The reason why diamonds are so very attractive and sparkling is due to the fact that total internal reflection occurs all the time in them! Diamonds have a critical angle of 250 and hence it is extremely likely that most of the light rays entering diamonds are bound to experience total internal reflection.

Total Internal Reflection and optical fibers

Ever wondered, how much time it would take to call someone in a different country 6000 miles away? With optical fibers it would take just a couple of seconds!

Uses of optical fibers


The greatest revolution in telecommunication was made with the discovery of Total Internal Reflection and its significant use in optical fibers.

Optical fibers are long glass rods through which laser light can be passed so that it gets Totally Internally Reflected and travels at the speed of light!

These fibers are astonishingly thin and have the capacity of carrying thousands of telephone calls simultaneously!

Note: The optical fibers must be of high purity or else, it would absorb light!


Optical fibers are also used in medicine along with endoscopes.

An endoscope is a device which is used by doctors to see inside the patient’s body, for the purpose of performing minor operations without the need for a major surgery!

Two bundles of optical fibers are passed through the patient’s body, one carrying light and the other carrying an image back to the physician.

The endoscope can also be equipped with small cutting tools that can perform minor operations (as stated above.)

PHET Simulation Explaining Total Internal Reflection!

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