Huygens’ wave theory of light:
In 1678, Dutch physicist Christian Huygens proposed a theory to explain the wave nature of light. This theory is called Huygens’ wave theory of light.
According to wave theory of light, a source of light sends out disturbance in all directions. When these waves carrying energy reach the eye, they excite the optic nerves and the sensation of vision is produced.
Main postulates of Huygens’ wave theory:
i. Light energy from a source is propagated in the form of waves: The particles of the medium vibrate about their mean positions in the form of simple harmonic motion. Thus, the particles transfer energy from one particle to its neighbouring particle and reach the observer.
ii. In homogeneous isotropic medium, the velocity of wave remains constant: Speed of the wave is not affected because density and temperature of isotropic medium are same throughout.
iii. Different colours of light waves are due to different wavelengths of light waves: Each wave has its own wavelength. As the wavelength changes, its colour and frequency also changes. This is indicated by change in the colour.
iv. The material medium is necessary for the propagation of wave: Periodic disturbance is created in the medium at one place which is propagated from that place to another place. The medium only carries disturbance and hand it over to the next particle.
To explain the propagation of light waves through vacuum, Huygens suggested the existence of a hypothetical medium called ‘luminiferous ether’