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Plastic scale as a prism


A plastic scale can be used as a prism. One surface of the scale is flat and the other surface is tapered near the edge. The two surface near the edge are inclined to form a plastic prism. All the actions of a prism, namely deviation of light beam, the existence of minimum deviation, dispersion etc can be shown using the scale.


Put the laser torch on a table and send the light beam to hit a wall or a screen placed at a distance of about 2 metre. Hold a plastic scale in your hand so that its length is vertical, the flat surface faces the laser torch and is perpendicular to the beam. Thus the scale is in vertical position and the lower end of the scale is resting on the table. Keep it somewhat away from the path of the laser beam so that you receive the direct beam on the wall.

Now slowly slide the scale towards the beam. As soon as it intersects the beam, the spot on the wall shifts, showing that the beam is turned by the scale. If you slide the scale further, the tapered position goes out and the spot comes back to the original position. The portion of the scale intersecting the beam is like a rectangular slab as both surfaces are parallel to each other.

Bring the tapered portion of the scale again in the path of the beam. The spot on the wall shifts due to deviation. Now rotate the scale at its place, in a way that keeps the tapered portion cutting the beam, but changes the angle of incidence. The spot will either go away from its direct beam position or will go towards it. In the first case the deviation is increased and in the second case it is decreased. Rotate the scale to decrease the deviation as much as you can. See that you can not reduce it beyond a minimum. Thus you get the minimum deviation position.

When the light beam goes through the prism and deviates, the spot on the wall becomes diffused. This may be partly due to diffraction from the edges, and partly due to roughness of the scale prism. However this does not affect our conclusions.

You can encourage students to perform the experiments in the informal lab and measure the angle of minimum deviation. One way is to measure different distances and use the properties of triangle. They should then measure the angle of the prism, that is, the angle between the flat part and the tapered part of the scale. Though it is not very straight forward, it is not difficult as well. The student should be encouraged to come out with a good method to measure this angle. They can then calculate the refractive index of their scale.


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

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