Concepts of Physics

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

Ray Tracing and Verification of the Snell's Law Using Rectangular Glass Slab


The Snell's laws of refraction are,

  1. The incident ray, refracted ray, and normal lies in the same plane.
  2. The angle of incidence \((i)\), angle of refraction \((r)\) and the refractive index of the medium (\(\mu\)) are related by \({\sin i}/{\sin r}=\mu\).

We verify the Snell's law in this experiment. The glass slab produce lateral shift of the incident ray.


Rectangular glass slab, white paper, protector, pencil, scale, pins.



  1. Fix a sheet of white paper on a thermocole sheet. Place the slab at its middle. Draw the boundary of the slab, and draw a line RP to meet one of the longer boundaries at P, at an angle (say 30 degree). Fix two pins A, B vertically on this line about 10 cm apart. Look at the image of the pins from other side of the slab. Now fix a pin C such that it appears to be in a straight line with the image of A and B. Fix another pin D (at least 10 cm from C) such that all four pins appear to be in straight line.
  2. Remove the pins and join by a straight line the points where the pins C and D were inserted. Extend this line to meet the boundary of the slab at Q. Join PQ. The lines RP, PQ, and QD represent the directions of the incident ray, the refracted ray within the slab and the emergent ray after the second refraction respectively.
  3. You will find that the QD is parallel to RP. Also, it is shifted sideways from the direction of RP. Note that the incident ray bent towards the normal at P, as it moved from the optically rarer medium (air) to the optically denser medium (glass). At Q, the ray going from the optically denser medium (glass) to the optically rarer medium (air), bent away from the normal at Q.
  4. Measure angle of incidence and angle of refraction and calculate \(\mu\).
  5. Repeat above steps of incident angle of 45 degree and 60 degree.


Write results in tabular form.