As per evidence from the wave theory of light, the first introducer or the host of the wave theory of light, Huygen, consider light to be a mechanical wave and hereby he states that each point on a luminous body oscillates periodically emitting out waves in all directions in a hypothetical medium (which do not really exist in real world) which was named as ether. If the medium is homogeneous and is isotropic in nature, ether particles located at equal distance from the source, at any instant of time are in the constant phase (or the same state of vibration).

Huygen’s proposal remained in dump for about a 100 years because particles theory as explained by Newton was accepted by the scientific community with great faith at that period of time but got its valued status when Sir Thomas Young seriously challenged the writings of Newton with the introduction of his double slit experiment in 1801 which revolutionise the field of Optical Physics.

The Propagation of light in a medium means the propagation of wavefront. As the ray represents the direction in which the disturbance travels in an isotropic medium ( a medium which has properties uniform in all the directions and are direction independent is stated as an isotropic medium), it must be perpendicular  (at 90°), to the wavefront.

Wavefront

A continuous locus of all such particles of the medium which are oscillating in the same phase at instant of time, is called wavefront and it is also defined as the source of constant phase.

For e.g. let’s consider throwing off a piece of stone in the lake, when we drop the stone in the water, waves spread out in all directions from the point of impact. At any instant of time ,all the collections of points on the circle where disturbance is maximum vibrates in the constant phase and this is because of the fact that they are at the same distance from the point of impact or the source.

Different Types of Wavefront

1. Spherical wavefront: When source is a point object.
2. Cylindrical wavefront: When the source of light is linear in the shape.
3. Plane wavefront; When the source of light is a plane.

Huygen’s Principle

As per  the general statement of Huygen’s principle “each point on a wavefront is a source/origin of secondary waves or wavelets, which add up to give wave fronts at any later time”.

It can also be stated as in its most broad and general structure as follows:

“Various focuses on any arbitrary surface when reached by a wavefront become a secondary source of light radiating auxiliary wavelets.

This statement cited by Huygen gets its underlying foundations from the accompanying presumptions:

1. Each point on the wavefront goes about as a new fresh source of another aggravation called secondary wavelets or waves.
2. The secondary wavefront or wavelets spread out uniformly in all directions with velocity of light in the given medium.
3. The new wavefront at any future time can be concluded by the forward envelope coverings of the secondary wavelets at that instant of time.

Drawbacks/Limitations of Huygen’s Principle

Huygen’s principle didn’t provide any scientific evidence to describe and deal with the absence of  back waves. To explain the absence of a back wave, Huygen argued that the amplitude of the wave is maximum in the forward direction and zero in the backward direction, but the given statement was not scientifically satisfactory.

Conclusion

• A ray of light constitutes an angle of 90° i.e. perpendicular to the wavefront
• The time taken for light to travel from one wavefront to another is constant 
• The energy of the wave travels in the direction perpendicular to the wavefront
• Huygen’s principle proves very fruitful in predicting and explaining numerous crucial characteristics of light as: 
  • Reflection of spherical wavefront at plane surface
  • Reflection of plane wavefront at plane surface
  • Refraction of plane wavefront at plane surface.