Refractive Index and Snell's Law

Light that hits a gem surface may be:

Reflection: the light is bounced off the surface: the angle of incidence equals the angle of reflection.

Refraction: light passes into the gem: the light path will change because the density of the stone is different from that of air. As light passes into the stone, it slows down. This is what causes its path to change. The bending of light as it passes into (or out of) the stone is known as refraction

The refractive index, abbreviated RI, is a measure of how much the light is refracted by a particular stone. Note that light is bent when it passes between air and water, or any two other mediums!

• The amount that light is bent is measured relative to the normal to the surface of the gem.

The amount of slowing of light depends upon the number, size, and arrangement of atoms (mineral composition and structure; thus, some minerals affect the speed and path of light as it passes through the stone more than others).

THUS, RI is a very useful diagnostic quantity.

Use of refractive index in gem identification is discussed in a later lecture. A value is measured and compared to values tabulated. It should be stressed that RI should be used in conjunction with other measurements in determining the identity of a gem.

More detail than is needed by EPS2 students:

The velocity of light in vacuum is 300,000 km/sec. As light passes into the gem it slows down. The refractive index measures this: RI=velocity of light in a vacuum divided by the velocity of light in the sample.

Snell's law - Not required for students of EPS2 --

Snell's law is used to calculate how much refraction occurs. This is determined from the refractive index (RI) and the angle of incidence (i).

RI=Sin i / sin r

Note that the amount light slows down depends upon its wavelength. This phenomena is called dispersion. We will talk about this later.