Suggested practical

Investigation of refraction of light through a glass block

Aim of the experiment

To investigate the refraction of light through a glass block.


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 a ray box, slit and lens so that a narrow ray of light is produced.
  2. Place a 30 centimetre (cm) ruler near the middle of a piece of plain A3 paper. Draw a straight line parallel to its longer sides. Use a protractor to draw a second line at right angles to this line. Label this line with an 'N' for 'normal'.
  3. Place the longest side of a rectangular acrylic polymer block against the first line. With the normal near the middle of the block, carefully draw around the block without moving it.
  4. Use the ray box to shine a ray of light at the point where the normal meets the block. This is the incident ray.
  5. The angle between the normal and the incident ray is called the angle of incidence. Move the ray box or paper to change the angle of incidence. The aim is to see a clear ray leaving the opposite face of the block.
  6. Using a pencil on the paper, mark the path of:
    • the incident ray with a cross
    • the ray that leaves the block with two crosses - one near the block and the other further away
  7. Remove the block. Join the crosses to show the paths of the light rays.
  8. Repeat steps 2 - 7 for a rectangular glass block.
  9. Measure the angle of incidence and angle of refraction for each block.


Record results in a suitable table.

AttemptPolymer blockGlass block
Angle of incidence (°)Angle of reflection (°)


Compare the angle of incidence with the angle of refraction for each block.


The light rays should not obey the law of reflection. What do the results show instead?


HazardConsequenceControl measures
Ray box gets hotMinor burns Do not touch bulb and allow time to cool
Semi-dark environmentIncreased trip hazardEnsure environment is clear of potential trip hazards before lowering lights