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Fig.1: Refraction of a light ray

Refraction is the change of direction in which light travels as it passes from one substance to another that has a different optical density (as from air into a gemstone).

Optical density is a property that 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 light to bend (refract), as can be seen when a stick is put in a pond. This bend is referred to as refraction. Refraction is based on Snell's laws of refraction (see below).

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 (like a gemstone), it will hit the surface at an angle to an imaginary line named the normal (NO). It will then partially enter the stone (other parts will be reflected). 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 (or less dense) medium such as air will bend away from the normal. The angle at which light is now refracted out of the stone is the same as the angle of incidence, which means it will continue along the same path as on incidence (when it entered the stone).

A gemstone’s index of refraction depends on wavelength. White light is composed of 7 spectral colors (Red, Orange, Yellow, Green, Blue, Indigo and Violet), each of which travels at different wavelengths and thus at different speeds. Therefore, the refractive index will be different for each of those colors.

In gemology, yellow light is used as the main source for measuring the refractive index of a gemstone. Yellow light was chosen because in the early days of gemology it was easily produced by salt (sodium) in a flame and was an inexpensive means of producing monochromatic light. The wavelength of sodium light lies at 589.6nm, 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 abbreviated as RI (refractive index).

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

The math behind Snell’s laws of refraction

Refraction follows the laws of Snell, which state that:

  • 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 wavelength. This ratio is known as the index of refraction.
<math>Index\ of\ refraction = \frac{\sin i}{\sin r}</math>
  • 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. As the index of refraction also depends on the velocity of light, one could write:

<math>n = \frac{velocity\ of\ light\ in\ air}{velocity\ of\ light\ in\ medium}</math>


<math>\mathbf{c} = \mathbf{l}\mathbf{n}</math>

Velocity (c) of light is dependent on wavelength (l) and frequency (n). Frequency is the number of crests that pass a given point in one second and is expressed in hertz (Hz).

The frequency of light is the same in every medium. This means that if the velocity of light changes, the wavelength must also change. As a consequence, light traveling from air into an optically denser medium will travel at shorter wavelengths inside that medium. Fluorescence is the exception to this rule because it emits light at longer wavelengths (Stokes' Law) caused by a loss of energy rather than by a change in velocity.

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