Refractometer

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The refractometer is one of the most important tools in a gemological laboratory. It indicates (not measures) the refraction index of a gemstone, which often gives vital clues to the identity of a gemstone.

Although one would expect a refractometer to measure the refraction of a gemstone, this is not the case. Instead it is based on a unique optical phenomenon named Total Internal Reflection (or TIR).

Basic

Construction of a gemological refractometer

Total Internal Reflection

Inside the refractometer: Total Internal Reflection

When light travels from an optically denser material (with higher index of refraction) to an optically rarer material (with lower index of refraction), all light that reaches the boundary of the two materials will be either reflected inside the denser material or refracted into the rarer material, depending on the angle of incidence of the light.

For every two media in contact, the dividing line where either the ray of light is reflected or refracted is fixed and can be calculated. This dividing line is named the critical angle (ca). On the left you find an image showing the critical angle as the red line.
When light reaches the boundary of the two materials at an angle larger than this critical angle (the blue line), the ray of light will be totally reflected back into the denser material. Light reaching the boundary at an angle smaller than the critical angle will be refracted out of the denser medium, into the rarer medium (the green line). All light travelling precisly on the critical angle will follow the path of the boundary between the two materials.

The standard gemological refractometer can make use of this phenomenon because the reflected rays of light will appear light on the scale, whilst the refracted rays are not visible (therefor appear black). The light/dark boundary shown on the scale of the refractometer is a visible representation of the critical angle.