Dispersion and Total Internal Reflection

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Dispersion and Total Internal Reflection

Why can you never find the pot of gold at the end of a rainbow?

White light is made up of lots of different coloured light. Rainbows are made by raindrops splitting up the white light into the separate colours. This is caused by refraction.

You will know from the sections above that when light waves enter water they slow down and bend. Different coloured light has different frequencies. The higher the frequency of the light wave the more it is slowed down and the more it is bent.

Dispersion and Total Internal Reflection

Red has a low frequency so is only bent a little. Violet has a much higher frequency so is bent more.

This is refraction taken to an extreme. Press the button below to see the three possible situations:

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As the angle of incidence increases the angle of refraction increases until it gets to 90 degrees. At this point the angle of incidence is at the critical angle. If the angle of refraction gets any bigger the wave bounces off the surface of the block and reflects back into the block. This is called total internal reflection. When this total internal reflection happens the light wave obeys all the normal rules of reflection.

In fact, whenever light travels through glass, Perspex or water into air a small amount of light energy is reflected back into the block, by the boundary. It only becomes very noticeable when the angle of incidence is greater than the critical angle and TIR happens.

This strange occurrence has now become extremely useful. Light can be made to bounce along long glass fibres, reflecting off the inside walls as it travels along, until it emerges at the other end. This is repeated total internal reflection.

Dispersion and Total Internal Reflection

These fibres are called optical fibres. They are used in medicine, in endoscopes, to see inside patients. Optical fibres are also used for communications; information is carried as pulses of light along the cables.

Dispersion and Total Internal Reflection

Total internal reflection can also be used in periscopes and reflectors.Right-angled prisms can be used to make the light change direction: 

Dispersion and Total Internal Reflection
Dispersion and Total Internal Reflection

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