Concepts of Physics

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

Refractive Index of a Glass Slab using Parallax Method


To find refractive index of a glass by measuring apparent depth/real depth.


Parallax: If two objects A and B are separated from each other and we see from a position C along line AB, it appears to us that two are touching each other and we are not able to see them as separate objects. However, if we shift our eyes towards left, the two will look separated. If we do not have depth perception, object A will look towards left and B towards right. Similarly, if we shift the eye towards right, the object appears separated, A towards right and B towards left. If the two objects A and B are actually at same place then they always appear in contact wherever you move your eyes.

The apparent depth \(h_a\) and real depth \(h_r\) are related to the refractive index of the medium by, \(\mu={h_r}/{h_a}\).


Rectangular glass slab, playing cards, scale.


  1. Draw a line on a sheet of paper. Place it on a table and put a transparent glass slab over it. The longest side of the slab should be vertical.
  2. Mark similar line on another paper sheet. Put some blank cards (sheets) on the table adjacent to the slab and on the top of this stack put the marked sheet. Looking from the above, the two lines should be in the same line.
  3. Insert or remove sheets from the stack to adjust the height so that no PARALLAX remains between the two lines, one as seen through the slab and other on top of the stack. In this situation, the depth of the stack is same as the image of the line formed by the slab.
  4. Measure the apparent depth (\(h_a\)) and real depth \(h_r\) of the line. Calculate the refractive index of the glass by using the formula given above.