# Difference between revisions of "Refraction"

## Basic

Fig.1: Refraction of a light ray

Refraction is the change in direction in which light travels, when it passes from one substance to another that have different optical densities.

Optical density is a property which manifests itself in the slowing down of light, i.e. the higher the optical density, the lower the velocity of light. This change in velocity causes the light to bend (refract), as can be seen when a stick is put in a pond.

Refraction is based on Snell's laws of refraction:

• The sine of the angle of incidence has a constant ratio to the sine of the angle of refraction, for any two given substances that are 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.

As stated earlier the index of refraction depends on wavelength.
White light is composed of its 7 spectral colors (Red, Orange, Yellow, Green, Blue, Indigo and Violet) which each travel at different wavelenghts and thus at different speeds. Therefor the refractive index will be different for each of those colors.
In gemology we use yellow light as the main source for the measurment of the refractive index of a gemstone. This yellow light was chosen as it was easily produced by salt (sodium) in a flame and an inexpensive means of producing monochromatic light.
The wavelength of sodium light lies at 589.6nm and this is known as the Fraunhofer D-line.

When n is used to describe the index of refraction, we use nD to indicate the refractive index when measured with sodium light. This is commonly also abbreviated with RI (refractive index).

The instrument of choice to measure the refractive index of a gemstone is the refractometer.

$V = \lambda\ .\ f$