The phase variation can be minimized by stacking two waveplates that differ by a tiny amount in thickness back-to-back, with the slow axis of one along the fast axis of the other.
12.
If the input polarization is parallel to the fast or slow axis, then there is no polarization of the other axis, so the output polarization is the same as the input ( only the phase more or less delayed ).
13.
A linear retarder has a slow axis and a fast axis, and the two axes are orthogonal to each other . Polarized light travels at higher speed when its electric field vector is aligned with the fast axis of a retarder.
14.
Negative uniaxial crystals ( e . g . calcite CaCO 3, ruby Al 2 O 3 ) have " n e " o " so for these crystals, the extraordinary axis ( optic axis ) is the fast axis whereas for positive uniaxial crystals ( e . g . quartz SiO 2, sellaite ( magnesium fluoride ) MgF 2, rutile TiO 2 ), " n e " > " n o " and thus the extraordinary axis ( optic axis ) is the slow axis.
