Prism dispersion (2D bench)

What you see

The prism bench seen from above: a 60 degree prism, a point source on the left, a screen on the right. Every colour is traced separately with its own refractive index, so white light fans out into a spectrum.

Snell's law

At each face n1 sin(theta1) = n2 sin(theta2). When light inside glass meets a face beyond the critical angle asin(1/n) it is totally reflected instead of leaving.

Dispersion

The refractive index comes from the three-term Sellmeier equation n^2 = 1 + sum B_i l^2 / (l^2 - C_i), with coefficients for N-BK7 crown glass, fused silica and sapphire. Violet light sees a higher index and bends more than red.

Minimum deviation

Rotating the prism changes the deviation; it is smallest when the ray crosses symmetrically, where delta_min = 2 asin(n sin(A/2)) - A. The right-hand graph measures it with the ray tracer and compares it with this formula.

Simplifications

Reflection losses (Fresnel) and polarisation are not modelled; the source is a single thin ray, not a beam. This 2D page uses the same physics module as the 3D page, so both give identical numbers.