How this calculator works
For a periodic wave, phase speed equals wavelength times frequency. Vacuum light travels at exactly 299792458 m/s. A transparent medium changes wavelength through its phase refractive index while frequency stays the same at a stationary boundary. Use a known index at the frequency of interest if the medium is dispersive. This calculation does not use group speed for pulse arrival times. Wavelength is entered in nanometres so visible-light values are convenient.
Formula
f = c/(n lambda); lambda = c/(n f); vacuum wavelength = c/f. For vacuum, n = 1.
Worked example
Light with vacuum wavelength 500 nm has frequency 5.99584916e14 Hz. In a medium with phase index 1.5 the same frequency has wavelength about 333.333 nm.
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Frequently asked questions
Does frequency change when light enters glass?
At a stationary interface the frequency stays fixed; speed and wavelength change.
What is the speed of light used here?
The exact SI value 299792458 m/s, as listed in CODATA 2018.
Is the index a constant for all colours?
Usually not. Enter the phase index for the wavelength or frequency being studied.