# Refractive index of Minerals

When a ray of light passes from air into a denser medium such as glass, it gets refracted. In the glass the light travels with a lesser velocity than in air and deviates from its previous path. The amount of deviation depends on the angle of incidence and the relative velocity of light in the two media.

The refractive index (n) of a mineral can be expressed as a ratio of the velocity of light in the air (V1) and its velocity in the minerals (V2). V1 is the speed of light (c), while V2 velocity of light in the secondary medium (v).

V1/V2 or

n = c/v

The velocity of light in air is generally considered equal to 1, therefore, becomes equal to 1/V2. This shows that the refractive index of a mineral varies inversely with the velocity of light in it.

For a particular mineral, the relationship between the angle of incidence (i) and the angle of refraction (r) is given by Snell’s law. This law states that the ratio of the sine of incidence angle to the sine of the refracted angle is a constant. This constant is called the refractive index (n) of the mineral.

n= sin i /sin r

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