LIGHT WAVES Refraction.
Refraction
Waves travel at different speed through different mediums, hence if a wave passes from one material into another at any angle other than
perpendicular to the interface, it will change direction. This effect is known as refraction.
The amount of refraction depends on the relative speeds of the wave in the different material, for instance in the diagram oppposite
ci / cr = sin( i ) / sin( r )
where c is the speed of the wave. This ratio of sin( i ) : sin( r ) is the refractive power of the combination of materials. Refractive indexes
are defined with one material being a vacum. i.e if the top material above is a vacum the refactive index of the bottum material n = sin( i ) / sin( r )
This can then be compared with any other material such that ni / n r = sin( i ) / sin( r ).
Notes:
- If the angle of incidence is such that the angle of refraction is 90o, then this is the critical angle of the material, and light is reflected back into the material.
- In an optical fibre two materials of different refractive indexes are combined so that the angle of incidence for light inside is greater than the critical angle and total internal reflection occurs.
Hence light can be passed down the fibre.
Rob Coles 20 May 1996 10:34:24