Concepts of Physics

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

Light Does Not always travel in a straight line


To see the path of light in an inhomogeneous transparent medium.


We all know that in a homogeneous medium, light travels in straight lines. There is a very easy way to make an inhomogeneous transparent medium in which light seems to follow a curved path.


A glass or plastic container with flat walls, concentrated salt solution, water, a syringe, a laser torch.


  1. Pour some amount of salt solution in the container. Let it settle down for a minute or so.
  2. Fill water in a syringe with its needle in place. Hold it carefully with the end of the needle gently touching one of the walls of the container.
  3. Gently push on the piston so that water comes out very slowly and trickle down the wall.
  4. Water will gently sit on the salt solution with some mixing at the interface. Send laser beam into the container through a wall so as to allow it to enter the liquid part horizontally
  5. Adjust the height of the torch so that it enters the liquid at the interface area. With little adjustment you will see that the path of the laser is like a circular arc.


The salt solution has higher refractive index than water. At the interface, due to diffusion of water into heavier salt solution, there is a vertical gradient \(\mathrm{d}\mu/\mathrm{d}h\) of refractive index. It may seem strange that if light enters horizontally and bend, while horizontally there is no variation in refractive index. It is easy to understand if you invoke Fermat's Principle.