Theory and evaluation
The velocity of the light travelling in the direction of the optic
axis of a double-refracting crystal has the same value, co,
whatever the direction of its plane of polarisation. When travelling
at right angles to the optic axis, polarised light has the
same velocity co, when the electric vector is perpendicular to
the optic axis (ordinary ray, see Fig. 2). If the electric vector is
parallel to the optic axis the light velocity c ± co (extraordinary
ray).