Total internal reflection

When light moves from a dense medium to a less dense medium (for example from water to air) it refracts away from the normal. The bigger the angle of incidence the nearer the light is to the surface as it comes out. If the angle is big enough the light cannot refract out and it is all reflected inside the dense material. That is called Total Internal Reflection (TIR for short) The angle of incidence when the TIR starts to happen is called the critical angle.

As the angle of incidence gets larger more light reflects. At an angle between the angle of incidence at A and the angle of incidence at B is the critical angle. That is the angle of changeover where all the light reflects.

To calculate the critical angle c we use the refractive index, symbol μ:

sin i/sin r  = 1/sin c = μ

Optical fibres

We make use of TIR to send light through optical fibres. Optical fibres are the strands of glass which carry the messages in all our major telephone networks. Because the fibre has a high density the light reflects inside until it comes out of the end. We use this idea to make decorations but much more important for sending data and video signals as pulses of light. The light passes down the fibre reflecting from the inside surface. None of the light comes out of the fibre. This is TIR.

Commercial optical fibres have a protective coating of a lower density material on the surface of the fibre. The light totally internally reflects on this inner boundary so that scratches on the surface do not spoil the reflections. The light reflects down the inner core.

The video lesson below

explains what an optical fibre by demonstrating a simple way of making one. The lesson goes on to show how it works and some of the things they can be used for.

The notes and video lessons below on other topics on the physics of light may be useful:

The Laws of Reflection - notes and video lesson

Drawing ray diagrams for lenses - notes and video lessons