Concepts of Physics

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

Trace the Path of a Ray through a Glass Slab


Ray tracing through a rectangular glass slab.


rectangular glass slab, pins, pencil, sheet of paper, scale



Let us trace the path of a ray of light through a transparent glass slab. Fix a sheet of white paper on a board. 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. 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.

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.

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.

You can repeat the experiment, once for the rays passing through the length of the slab and once, through the height. Verify that the lateral shift of the ray is proportional to the thickness of the material of the slab through which the ray passes.