Minimum deviation
= Minimum deviation
{title2=$\gamma_{\min}$}
{wiki}
For a homogeneous <optical prism> in air, with apex angle $\phi$ and <refractive index> $n>1$, minimum deviation occurs at the symmetric ray path. Then both internal angles are $\phi/2$ and both external angles are $(\gamma_{\min}+\phi)/2$, so <Snell's law> gives
$$
n=\frac{\sin[(\gamma_{\min}+\phi)/2]}{\sin(\phi/2)}.
$$
Measuring the deviation therefore determines the refractive index at the chosen <wavelength>.