Scheme shows how the wave plate works. Optical axis is parel to the surface of wave plate.
Marks used in scheme:

Thinest line shows the direction of the optical axis
lines with dots and/or strokes are light rays. Dots and strokes are symbols of direction of polarization
dots — polarization perpendicular to the plane of the image
stokes — polarization parel to the plane of the image
e — ekstraordinary ray
o — ordinary ray
interrupted line shows a wavefront
circles and elipses are some of partial waves, they are to show how Huygens' rule works here.
Own work
Sfu