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Refraction
When a wave passes from one medium to another, the angle of refraction and the angle of incidence are related according to Snell's Law:
Where v1 is the wave velocity in medium 1 and v2 is the wave velocity in medium 2. Notice also, that because v1 is not equal to v2 the wavelength in medium 2 is different from that in medium 1.
For light waves, the ratio v1 / v2 is called the Relative Refractive Index. If medium 1 is a vacuum and medium 2 is a transparent material, then the ratio is called the Absolute Refractive Index for that material and is given by     n = c / v m  where v m is the speed of light in that material. NB v m is always be less than c and so n is always greater than 1. The refractive indexes of some common materials are listed in the table below.
Material
Absolute Refractive Index
Air
1.0008
Water
1.330
Glass, soda-lime
1.510
Diamond
2.417
Ruby
1.760

In general, some of the light incident at an interface will be reflected and some will be refracted.

If the incident ray is in the medium of higher refractive index then the refracted angle is greater than the incident angle and as the incident angle is increased there will be an angle icritical at which the refracted angle is 90o. For any incident angles greater than or equal to icritical , there will be no refacted light at all and all the incident light will be reflected. This is called Total Internal Reflection.

The Critical Angle is given by the expression...
 sin icritical = n1 / n2

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