An introduction to…Optics

This is an experiment that is easy to do, but difficult to do well!

  1. Set up a ray box to give a single ray. Put the ray box and a 60° prism on a sheet of paper.

  2. Adjust the position of the prism and/or ray box so that the ray passes through as shown in the diagram below.

  1. Mark accurately, on the paper, the position of the sides of the prism and the incident and emergent rays. You may note that the emergent ray shows dispersion — splitting of the light into the colours of the rainbow. Always mark the same colour.

  2. Remove the prism and, as accurately as possible, complete the diagram to show the refracted ray inside the block, the continuation of the incident ray, the extension of the emergent ray and the normal (N-N).

  3. Measure the angles of incidence (i) and deviation (D).

  4. Record your results in a table.

  5. Adjust the prism and/or ray box to give a different angle of incidence and repeat the measurements.

  6. Repeat until you have at least 8 sets of readings with angles of incidence over as wide a range as possible. Note that at small angles of incidence the ray will be totally internally reflected inside the prism – adjust so that the ray does emerge from the correct side.

Plot a graph of deviation (y-axis) against angle of incidence (x-axis). Draw the best fit curve. You may decide at this point to go back and take some further readings to ensure that you can produce the best possible curve.

Determine the minimum deviation and the angle of incidence at which this occurs.