Wave Theory: According to Huygens, light is propagated in the form of waves, through a continuous medium. Huygens assumed the existence of an invisible, elastic medium called ether, which pervades all space. The disturbance from the source is propagated in the form of waves through space and the energy is distributed equally in all directions. Huygens assumed these waves to be longitudinal. Initially rectilinear propagation of light could not be explained. But the difficulty was overcome when Fresnel and Young suggested that light waves are transverse. The wave theory could satisfactorily explain all the basic properties, which were earlier proved by corpuscular theory and in addition, it explains the phenomena of interference, diffraction and polarization.
According to Huygens, the velocity of light in a denser medium is lesser than that in a rarer medium. This is in accordance with the experimental result of Foucault.