In 1678, Huygens proposed the waves theory of light. According to Huygens, light travels in the form of waves. These waves after emerging from the light source travel in all directions with the velocity of light. Since waves require a medium to travel. Huygens imagined an all-pervading medium called ‘luminiferous ether’. It was assumed that this hypothetical medium is weightless and can penetrate through matter. It has all properties necessary for the propagation of light waves.
According to Huygens principle, each point of the wavefront is the source of a secondary disturbance and the wavelets emanating from these points spread out in all directions with the speed of the wave. These wavelets emanating from the wavefront are usually referred to as secondary wavelets and if we draw a common tangent to all these spheres, we obtain the new position of the wavefront at a later time.
From the figure, the direction which the disturbance is propagated is called the ary. In a homogeneous isotropic medium, the rays are always normal to the wavefront.
The Huygens construction gives a backward wavefront also which is contrary to observation. This is because the intensity of the secondary of the secondary wavelets varies continuously from a maximum in the forward direction to zero in the backward direction.