Question

What is wavefront? Describe Huygen’s theory of secondary wavelets.

Answer 1

Hint: Dutch physicist Christian Huygens gave the first convincing wave theory for light. Mathematically simpler than the electromagnetic theory of Maxwell, it nicely explained reflection and refraction in terms of waves.

Complete step-by-step answer:
When a source emits waves, the waves spread out in all the directions. Wavefront is a surface where all the points of the wave on this surface are in phase. For example, one point on this surface is at maximum amplitude then all the points on this surface will be at their maximum amplitude and if one point on this surface is at its mean value then all the other points on this surface will be at their mean values. In simple words, we can say that all the points on the wavefront have the same nature.
Huygen’s theory of secondary wavelets states that each point on the wavefront is the source of the secondary wavelets which spread out in all directions with speed of the wave. Suppose there is a planar wavefront. According to the theory, every point on this wavefront acts as a secondary source and emits wavelets which spread in all directions with speed of the wave. If we make a surface tangentially to the wavelets , we will get a new position of the wavefront. This is how the wavefront travels forwards.
To understand it better, let us take an example of water. When you throw a stone on water, ripples are created in the water, which are actually waves spreading out in all directions. The point where the stone hits the water surface acts as a source of the disturbance. This disturbance spreads in the form of waves. If you pick up a single ripple, that is a wavefront (here it is a circular wavefront). Every point in the ripple acts as a secondary source of disturbance and they also cause a disturbance to spread in all directions. These disturbances are wavelets. If you draw a curve tangentially to these wavelets, you will get the next ripple. Then again, all the points this ripple act as a secondary source and the same process continues resulting into a number of ripples.

Note: A wavefront is not always a surface. It can also be a two dimensional curve. Like in the example of waves (ripples) formed in the water. Here, the wavefront is not a surface rather it is a circle. All the wavefronts (ripples) formed are concentric circles with the point, where the stone hit the water as the centre.