Concepts of Physics

IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

Principle of Optical Fibre


To study the principle of optical fibre.


Optical fibres are long synthetic fibres used to transmit light from one place to another. They form the backbone of modern communication systems. They work on the principle of total internal reflection.


A plastic bottle/cup, laser torch


Make a hole near the bottom of the bottle/cup. Add little soap in the water and fill the bottle with this water. Point the laser beam at the hole in horizontal orientation. The light travels along the water stream. Put a finger in the water stream and see a spot of light on your finger.



In the water stream, light strike the interface between water and outside air at an angle more than the critical angle. So it is reflected back into the stream. This reflected beam too strike the opposite surface at an angle more than the critical angle and is again reflected back. This process goes on and on i.e., multiple total internal reflections takes place.