Total Internal Reflection

When the angle of incidence is greater than the critical angle and the light is travelling from a dense to a less dense medium then total internal reflection occurs. This means that all of the light is reflected. Total internal reflection is therefore a more efficient method of refection than the use of mirrors, which reflect about 90% of light and is therefore particularly useful in optics.

The critical angle is related to the refractive index using the following equation:

The size of the critical angle depends on the material. Diamonds sparkle due to a lower critical angle than glass, therefore total internal reflection occurs more.

In the exam they will often specify a boundary. Note that when the question specifies the air/water interference, they are referring to light passing from water to air. In the same way that if they specify the water/air interference it is referring to light passing from air to water.

Multipath dispersion

Total internal reflection is used in optical fibres. They allow information to be sent extremely quickly and are particularly useful in communication. However this poses the problem of multipath dispersion. This is when light travelling along the shortest path arrives first therefore the pulse spread out. This results in the signal quality being reduced.

Multipath dispersion can be reduced by using a material with a high critical angle so a lot of light is lost therefore it all follows virtually the same path. However, this method does require amplification. The use of narrower fibres so all the light follows virtually the same path will too help to reduce multipath dispersion.