In 1678, Huygen proposed the wave theory of light. According to which light travels in the form of waves. These waves after emerging from the light source travel in all directions with velocity of light. This wave character of light successfully explained the phenomenon of polarisation, diffraction and interference. While it is failed to explain photoelectric effect. This was later explained by Einstein on the basis of Planck's quantum theory which states that light travels in the form of small bundles or packets of energy called photons.