Subject knowledge and
challenging concepts
- Refraction occurs when the density of the medium that waves are travelling through alters. It affects the speed and direction. Waves entering a denser medium will slow down and bend towards the normal (drawn at right angles to the boundary). The opposite happens when entering a less dense medium.
- Students will often accept that light bends when passing from one medium to another and will have encountered such examples as swimming pools appearing to be shallower than they really are. However, explaining why this should be is rather more problematical. We need to use the wave model and emphasise that what we represent as a ray may need to be thought of as a succession of waves.
- Waves entering a denser medium will slow down (so light travels slower in glass than in air) and if they approach the denser medium other than along the normal route (ie at right angles to the surface), one end of a particular wave will be slowed down before the other and this will result in a change in direction. The wavelength of the waves becomes less.
- This will need reinforcing; there are good computer simulations and other techniques for illustrating this. Ripple tanks are useful, although they can be tricky to use to produce convincing results. Generally speaking they are more successful if students know what they are looking for. A skilful demonstration may achieve more than a class practical. It is also possible to model the effect using �waves� of students marching from �air� into �water�.
- The extent to which light travels slower in a particular medium is known as the refractive index (n) and is calculated thus:
n = speed of light in air
speed of light in medium - For example, glass has a refractive index of 1.5n. A denser medium has a greater refractive index, will bend light by a greater amount and will slow it down more. Wave refraction gives a good context for developing a theoretical model that can then be applied to a number of situations. What is particularly important is that the model is best developed or explained in a context where individual waves can be seen and then applied to other situations where they can�t be.