Using Huygens's principle, construct the wave fronts and the propagation directions of the ordinary and extraordinary rays in a positive uniaxial crystal whose optical axis
(a) Is perpendicular to the incidence plane and parallel to the surface of the crystal;
(b) Lies in the incidence plane and is parallel to the surface of the crystal;
(c) Lies in the incidence plane at an angle of 45 ° to the surface of the crystal, and light falls at right angles to the optical axis.