in isotropic medium, a ray of light is perpendicular to the wavefront when the medium is anisotropic the rays are not always perpendicular to the wavefronts Huygen.s THEORY assumed that intensity of SECONDARYWAVE lets is not UNIFORM, but varies from a maximum in forwared direction to a minimum in BACKWARD direction ace to Huygen.s theory the intensity of secondary wavelets is uniform in both forward and backward directions