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An optical prism is a transparent object with flat, polished surfaces that bend light in a specific direction due to refraction. Two important properties of prism are (i) deviation by a prism and (ii) dispersion by a prism.

Ray tracing through prism

Let us trace a ray of light through a prism. Fix a sheet of white paper on a board. Place the prism on it and draw its outline ABC. Draw a line PQ that meets AC at Q, at an angle of about 30 degree to AC. Fix two pins K and L vertically on this line, about 10 cm apart. Now look at the image of the pins from the side BC of the prism. Fix a pin M such that it appears to be in a straight line, the other pins will disappear behind M. Fix another pin N (at least 10 cm from M) such that all four pins appear to be in a straight line.

ray tracing through prism

Remove the prism and the pins. Join by a straight line the points where the pins M and N were inserted. This line, SR, meet BC at R. Join Q and R by a straight line. The lines PQ, QR and RS represent the directions of the incident ray, the ray within prism and the emergent ray respectively.

Draw perpendiculars to AC and BC at Q and R respectively. Measure the angles \(i_1\), \(r_1\), \(i_2\) and \(r_2\). Also, measure the angle of deviation, i.e., the angle between PQ and RS.


  1. Refraction at Plane and Spherical Surfaces
  2. Plastic Scale as a Prism
  3. Deviation by a Prism
  4. Refractive Index using Hollow Prism
  5. Dispersion of Light

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