Wave Model of Light

Introduction to wave model of light

Up to the middle of the 17th century, it was believed that light consisted of stream of corpuscles, emitting by the light source and travelled outwards from the source in straight lines. This theory is known as the Newton’s corpuscular theory. However, after 1827, the experiments of Young and Fresnel on interference, and the measurement of the velocity of light in liquids by the Foucault demonstrated phenomena, which could not be correctly explained by corpuscles theory but could be explained by the wave theory of light.


Huygens wave theory of light


In 1678, Huygens proposed the wave theory of light. According to this wave theory, light travels in the form of waves. These waves after emerging from the light source travel in all directions with the velocity of light. As the wave requires the medium to travel, Huygens imagined an all-pervading medium called aluminiferous ether. It was assumed that the hypothetical medium is weightless and can penetrate through matter. It has all properties necessary for the propagation of light waves. Hence, it was assumed that the density of ether is very small and the elasticity is very large. Light waves travel in such a hypothetical medium. When these waves fall upon the retina of the eye, they cause the sensation of sight. Huygens proposed the geometrical construction to explain the propagation of a wave front in the medium and determined the position of the wave front after any interval of time. They are known as the Huygen’s principle.



Conclusion of Huygeng’s wave theory of light


Every particle of the medium situated on the wave front acts as a new wave source from which the fresh waves originate. These waves are called the secondary wavelets.

The secondary wavelets travel in the medium in all directions with the speed of the original wave in the medium.

The envelope of the secondary wavelets in the forward direction at any instant gives the new wave front at that instant.