Prism Deviation Calculator

About this calculator

When light passes through a triangular prism, it bends at both surfaces; at one specific angle of incidence the total deviation angle δ reaches a minimum, and at that symmetric point it relates cleanly to the prism's apex angle A and refractive index n through δ = 2·asin(n·sin(A/2)) − A — a rearrangement of Snell's law applied twice, once at each face.

This minimum-deviation condition is the standard method optics labs use to measure a material's refractive index precisely: by rotating a prism to find the angle of minimum deviation experimentally and then solving the formula for n, spectrometer measurements can determine refractive index to several decimal places without needing to know the light's exact path inside the glass.

Optics and photonics students verifying Snell's law in the lab, spectrometer operators calibrating prism instruments, and physics educators demonstrating dispersion all use this relationship, since it's one of the few refraction formulas that can be checked directly against a real, measurable prism.