By Zencalcy Editorial TeamLast reviewed About us
Because the refractive index changes with wavelength, a prism bends blue light more than red. The difference in deviation is the angular dispersion, which sets how widely a spectrum is spread for a given prism.
Colour spread from a thin prism.
Δδ = (n_blue − n_red) A
The thin-prism approximation treats deviation as δ = (n − 1)A. Δδ is the difference in deviation between two wavelengths, with A in radians for the strict form.
| Symbol | Meaning |
|---|---|
| Δδ | Angular spread between the two wavelengths. |
| n_blue, n_red | Refractive indices at the blue and red ends. |
| A | Apex angle of the thin prism. |
Δδ = 0.01 × 10° = 0.1°, or 6 arcminutes. Real dispersing prisms use larger apex angles and glasses with a bigger index spread.
Continue with the Brewster Angle Calculator, Camera Exposure Calculator, or the Coherence Length Calculator — or browse more optics and light tools and all physics calculators.
Dispersion power is the index difference divided by the mean index minus one. It compares how strongly two glasses separate colours independent of prism size.
Results are computed from the standard equations of classical mechanics. Unit conversions use the SI definitions published by NIST. Air resistance is ignored unless the calculator names a drag model.
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Content updated: September 2026