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Index of Refraction

Excerpt from Field Guide to Spectroscopy

The speed of light in a transmitting medium is less than that of the speed of light in a vacuum. The index of refraction, n, is the ratio of the speed of light in a vacuum, c, to the speed of light in a medium, c':

One consequence of this difference in speed is that when light goes from one medium to another at an angle, the propagation vector in the new medium has a different angle with respect to the normal.


This change in direction of propagation is called refraction. The relationship between the indices of refraction and the angles is

n1 sinθ1 = n2 sinθ2

This relationship is called Snell's law.

Indices of refraction are temperature-dependent and wavelength-dependent.

Indices of refraction of common materials*

material nmaterialn
flint glass1.55rock salt1.544
quartz1.544, 1.553diamond2.417

*for light having a wavelength of ~550-590 nm


D. W. Ball, Field Guide to Spectroscopy, SPIE Press, Bellingham, WA (2006).

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