# Difference between revisions of "Refraction"

## Basic

Fig.1: Refraction of a light ray

Refraction is the change in the direction in which light travels, when it passes between two media of different optical density (except when incidence is at 90° to the interface).

Optical density is a complex property which manifests itself in the slowing down of light, i.e. the higher the optical density, the greater the reduction in velocity. This change in velocity has the effect of altering the direction in which light travels.

Refraction is based on Snell's laws of refraction:

• The sine of the angle of incidence bears a constant ratio to the sine of the angle of refraction, for any two given media in contact, and for light of a given wavelenght. This ratio is known as the index of refraction.
$Index\ of\ refraction = \frac{\sin i}{\sin r}$
• The incident ray, the refracted ray and the normal all lie in the same plane.

The index of refraction is abbreviated with the letter n and as the index of refraction also depends on the velocity of light, one could write:

$n = \frac{velocity\ of\ light\ in\ air}{velocity\ of\ light\ in\ medium}$

In Fig.1 the angle of incidence is indicated with i and the angle of refraction by r.
When light travels from air to an optically denser medium (as a gemstone), it will hit the surface of the gemstone at an angle to an imaginary line named the normal (NO). It will then partially enter the stone (other parts will get reflected) and due to the slowing down of light inside the stone, it will bend (refract) towards the normal.

The opposite of this is also true. Light that travels from a gemstone into an optically rarer medium (as air) will bend away from the normal. The angle at which it is refracted out of the stone is the same as the angle of incidence.
So it will continue its path in the same direction as on incidence.