Prescribed practical 5: angles of incidence and refraction


A guide to carrying out a practical investigating angles of incidence and refraction

  • To use ray tracing to measure the angles of incidence and refraction when light is refracted by a glass block;
  • To demonstrate understanding that the angles of incidence and refraction are measured from a line at right angles to the glass surface known as the normal;
  • and to use the measurements taken to plot a graph of angle of incidence against angle of refraction to show that they are related but not proportional.

There are different ways to investigate refraction in rectangular blocks.

In this required practical activity, it is important to:

  • make, measure and record the angles of incidence accurately using a protractor;
  • observe and use a protractor to measure angles of refraction.


In this experiment:

Independent Variable is the angle of incidence.

Dependent Variable is the angle of refraction.

Control Variables are the material of the block, the shape of the block and the colour of the light.

Remember - these variables are controlled (or kept the same) because to make it a fair test, only 1 variable can be changed, which in this case is the angle of incidence.


Light is travelling from air to glass and so is refracted towards the normal.

However, as the angle of incidence increases the refracted light will bend from a bigger initial angle, and so the angle of refraction will also be bigger.


Low voltage power pack, a 12V ray box, a single slit comb, a rectangular glass block, a sheet of white paper, a protractor, a sharp pencil.

The light ray as it enters the block, is refracted slightly, and then leaves the block. The smallest angles between the light ray and the block as the ray enters and leaves are the same size.


  1. Set up the ray box and slit so that a narrow, bright ray of light is produced.
  2. Place the rectangular glass block on a sheet of white paper and draw around it carefully with a pencil.
  3. Remove the glass block. Use the protractor to draw a normal approximately 1/3 of the way along the longest side.
  4. Use a protractor to measure angles of incidence from this normal of 100, 200, 300, 400, 500, 600 and 700. Draw in the incident rays corresponding to these angles and label them A, B, C.... Record these angles of incidence in a suitable table.
  5. Carefully replace the block on the outline. Direct a narrow ray of light along the line marked A. This is the incident ray for the angle of incidence, i = 100.
  6. With the pencil mark two Xs to indicate the direction of the emergent ray. Mark the Xs as far apart as possible.
  7. Remove the block again. Join the Xs with a pencil line, drawn using a ruler. Extend this line back to the block. This is the emergent ray: label it 'A'.
  8. Use the ruler to join the incident and emergent rays together with a pencil line. This is the refracted ray. Carefully mark in the angle of refraction, r, between the refracted ray and the normal. Measure the angle of refraction with a protractor and record in the table.
  9. Repeat the procedure for each of the incident rays, recording angle of incidence and corresponding angle of refraction in the table.
HazardConsequenceControl measures
Ray box gets hotMinor burnsDo not touch bulb, allow time to cool
Semi-dark environmentIncreased trip hazardEnsure environment is clear of potential trip hazards before lowering lights

Possible error

The main cause of error in this experiment is the measurement of the angles of incidence and refraction.

This can be kept to a minimum by:

  • replacing the block carefully on its outline;
  • ensuring that the power pack is set to 12 V, so that the ray box is at maximum brightness;
  • doing the experiment in a dark room so that the emergent ray can be easily seen and marked.