When a wave hits the boundary between two media, for example a light beam travelling through the air which then enters glass, or water, then if the new medium allows it the light ray will continue through the new medium but its direction may change.


At the boundary, there are three things that could happen:


  • The light will simply be absorbed, such as shining a torch beam at a black or non-reflective surface.
  • The light will be reflected, ‘spectacularly’ (like a mirror reflection) or ‘diffusely’ (like the fuzzy glare off a sheet of white paper).
  • The light will enter the medium, ie: transmission occurs through the medium but the beam may be displaced from its original path, “bent” or “refracted” 



In this example, the light is entering a more dense medium (glass) from a less dense medium (air). The speed of the light ray will slow down, causing it to bend towards the normal.

The normal is a line drawn perpendicular to the plane of the boundary between the two media.


When the light ray exits the glass and re-enters the air, the change in media density causes it to speed back up, in this case the light ray bends away from the normal.



GCSE Practical work will involve the investigation of reflection and refraction, using a light box / ray box, white paper and a darkened room.


The example below was conducted using a red laser pen and a Perspex block.



This experiment was a 10 cm x 10 cm Perspex block on a sheet of A4 photocopying paper. The light source was a quite powerful red laser, and the beam produced has been overlaid with yellow

arrows to show that the speed of light through the Perspex medium did slow down a little bit as the beam bends towards the normal (the horizontal white dotted line). The angle of incidence and the angle of refraction were measured using a computerised screen protractor:




The actual precision of the measurements to 2 decimal places is questionable due to experimental setting up errors and it would almost certainly in this case be rounded to the nearest integral degree, in which case the angle of incidence would be 37° and the angle of refraction would be 23° showing a divergence of 14° towards the normal.


At the time of writing this document, the remainder of this section does not appear to be on the current UK GCSE syllabus, but it is interesting nonetheless.


The speed of light is determined by the medium (material) through which the light is travelling. Light travels faster in a vacuum than it does in any other medium. Light changes speed as it passes from one medium to another. This is called refraction.

The frequency of light does not change as it refracts.

The refractive index of a material is a measure of the change in the speed of light as it passes from a vacuum (or air as an approximation) into the material.

In the equation above:

  • n is the refractive index of the material
  • v1 is the speed of light in a vacuum
  • v2 is the speed of light in a material

The bigger the refractive index, n, the slower the light travels in that material - i.e. the smaller v2 is.

>> Questions <<