Plane polarised light $($ lambda $=5*10^-7$ m$)$ is incident on a quartz plate cut parallel to the optic axis. Find the least thickness of the plate for which the ordinary and extraordinary rays combine to form a plane$-$polarised light on emergence. What multiples of this thickness would give the same result? The indices of the refraction of quartz are mu$\_\{$c$\}=1\mathrm{.}5533$ and mu$\_\{0\}=1\mathrm{.}5442.$