PRISM
Snell twice per color — and at minimum deviation, the prism measures its own glass.
Use the simulation above to change the variables and play through the guided stages. The explanation below describes the default starting values; the simulation updates its explanation as you experiment.
Setup
White light meets a triangle of glass. The glass has an opinion about color — a slightly different refractive index for every wavelength — and a prism is the shape that turns that private opinion into a public rainbow.
Cauchy
Cauchy’s relation: n falls as wavelength grows, so blue light meets denser glass than red. The difference is a third digit — but Snell’s law is applied twice, once at each face, and small disagreements applied twice do not stay small.
Solve
Trace each color through: refract in, cross the glass, then test the second face. The total deviation is δ = θ₁ + θ₂ − A for every color that escapes; any color beyond its critical angle reflects internally while the others continue.
Split
The construction, ray by ray. One white line enters and three colored paths separate inside. At the exit each color either refracts out or reflects by total internal reflection; the diagram and readouts preserve every surviving ray.
Audit
The audit: Snell exact at both faces for every color, deviation strictly ordered blue over red, and the crown jewel — at minimum deviation the passage is symmetric and n = sin((A+δ_min)/2)/sin(A/2), recovered to machine precision. The prism does not just split light; it measures its own glass.