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The wavelength of light, λ, in a medium depends upon the refractive index of that medium (see ‘Refractive Index’ entry). The wavelength of light in a vacuum is often referred to as λ₀ or sometimes λ_vac.

Frequency is unchanging in a photon or optical wave, though the wavelength will change depending on the medium, giving rise to a variable phase velocity, even though the speed of light remains constant.

Related Terms: Color, Emission Spectra, Photon