Retardance
= Retardance
{title2=$\Delta$}
The relative phase accumulated by orthogonal eigenpolarizations is $\Delta=2\pi L(n_2-n_1)/\lambda_0$. A <quarter-wave plate> has relative phase $\pi/2$ modulo $2\pi$, while a <half-wave plate> has $\pi$ modulo $2\pi$.