How this calculator works
The absolute refractive index is the vacuum speed of light divided by phase speed in a material. Relative index n2/n1 also equals v1/v2 and sets the direction change in Snell law. The phase index used here is distinct from group index, which controls pulse envelope travel. Approximate material presets are useful for visible-light examples but are not dispersion models.
Formula
n = c/v; v = c/n; nrelative = n2/n1 = v1/v2; c = 299792458 m/s exactly.
Worked example
An index of 1.5 gives a phase speed of 199861638.667 m/s. Media with indices 1.5 and 2 have relative index 4/3.
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Frequently asked questions
Can an index be below one?
Phase indices below one occur in some frequency ranges. Phase speed alone does not represent information speed.
Which way is the relative ratio defined?
This page consistently uses n2/n1. Reversing the media takes the reciprocal.
Can I use the preset for any wavelength?
No. Use a measured or tabulated index at the wavelength and temperature of interest.