(Physics Ch-9) 3. Total Internal Reflection

  • Total internal reflection is the phenomenon of reflection of light into a denser medium from an interface of the denser medium and the rarer medium.

  • Two essential conditions for total internal reflection:

  • Incident ray should travel in denser medium and refracted ray should travel in rarer medium.

  • Angle of incidence (i) should be greater than the critical angle for the pair of media in contact.

  • Relation between refractive index and critical angle (C)

When, i = Cr = 90°

Applying Snell’s law at A2,

μb sin C = μb sin 90° = μa × 1


  • Some applications of total internal reflection

  • Brilliance of diamond − The critical angle for diamond-air interface is 24.4°. The diamond is cut suitably so that light entering the diamond from any face falls at an angle greater than 24.4°, suffers multiple total internal reflections at the various faces, and remains within the diamond. Hence, the diamond sparkles.

  • Mirage → It is an optical illusion in which an object such as tree appears to be inverted.

This happens due to uneven heating of the different layers of air due to which density and refractive index of air goes on increasing slightly with height above the surface of earth.

As a result of this, light from a tall object, such as a tree, passes through a medium whose refractive index decreases towards the ground. Thus, a ray of light undergoes total internal reflection. To a distant observer, the light appears to be coming from somewhere below the ground.

  • Prism − Prism is designed to bend light by 90° or 180° and to make use of total internal reflection.

  • Optical fibres

Optical fibres are extensively used for transmitting audio and video signals through long distances. These make use of the phenomenon of total internal reflection. When a signal in the form of light is directed at one end of the fibre at a suitable angle, it undergoes repeated total internal reflections along the length of the fibre and finally comes out from the other end.


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