Prism (optics)

For a homogeneous optical prism in air, with apex angle $\varphi$ and refractive index $n>1$, minimum deviation occurs at the symmetric ray path. Then both internal angles are $\varphi /2$ and both external angles are $({\gamma }_{\min }+\varphi )/2$, so Snell's law givesMeasuring the deviation therefore determines the refractive index at the chosen wavelength.