Plane of polarized light

An experiment showed that two pieces of Iceland spar, if lined up parallel, allow unpolarized light to pass through unimpeded, while if one piece is rotated, the plane of the polarized light must be turned for the light to pass through again.
 Jean-Baptiste Biot France 1815 Plane of polarized light

An experiment showed that two pieces of Iceland spar, if lined up parallel, allow unpolarized light to pass through unimpeded, while if one piece is rotated, the plane of the polarized light must be turned for the light to pass through again.