Dispersion (optics)

Dispersion is the wavelength dependence of a material’s refractive index, which causes different colors of light to refract by different amounts and separate.

Refractive index: A dimensionless measure of how much light slows and bends in a material. Its variation with wavelength is the material property that produces dispersion.
Achromatic lens: A lens assembly designed to bring two selected wavelengths to a common focus. Combining glasses with different dispersions reduces color fringing in images.
Wavelength: The spatial period of a wave, measured between successive points in the same phase. Visible wavelengths correspond to colors that refract by different amounts in dispersive materials.
Group velocity dispersion: The variation of a pulse’s group velocity with frequency, causing different frequency components to travel at different speeds. It describes pulse spreading from frequency-dependent propagation, rather than simply angular color separation.
Snell's law: An equation relating incidence and refraction angles to the refractive indices of two media. Combined with wavelength-dependent refractive index, it predicts each color’s refracted angle.
Apochromatic lens: A lens corrected to bring three wavelengths to a common focus, with reduced secondary spectrum. It corrects dispersion across more wavelengths than a basic achromatic lens.
Visible spectrum: The range of electromagnetic wavelengths detectable by human vision. It is the familiar band of colors separated in visible-light dispersion.
Anomalous dispersion: A spectral region where refractive index decreases as wavelength increases. It contrasts with the usual visible-range trend of refractive index decreasing with increasing wavelength.
Cauchy's equation: An empirical formula approximating a transparent material’s refractive index as a function of wavelength. It quantifies how refractive index changes across visible wavelengths in transparent materials.
Spectrometer: An instrument that measures properties of light across a range of wavelengths. Dispersive elements can spread incoming light so a spectrometer measures its spectrum.

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