Define **Huygens **Principle

### Asked by bmoumita4141 | 1st Mar, 2020, 07:16: AM

Expert Answer:

### Huygens principle is a geometrical construction that helps us to determine the shape of the
wavefront at an instant of time from the given the shape of the wafefront of some earler time .
Let us consider a diverging wave and let F1F2 represent a portion of the spherical wavefront at t = 0
as shown in above figure. Now, 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.

Thus, if we wish to determine the shape of the wavefront at t = τ, we draw spheres of radius vτ from each point
on the spherical wavefront where v represents the speed of the waves in the medium.
If we now draw a common tangent to all these spheres, we obtain the new position of the

wavefront at t = τ. The new wavefront shown as G1G2 in figure is again spherical with point O as the centre.

In the above model, backward wave will be absent and this is justified using rigorous wave theory.
In a similar manner, we can use Huygens principle to determine the shape of the wavefront for a plane wave
propagating through a medium as shwon in figure given below.

Thus, if we wish to determine the shape of the wavefront at t = τ, we draw spheres of radius vτ from each point

wavefront at t = τ. The new wavefront shown as G1G2 in figure is again spherical with point O as the centre.

In the above model, backward wave will be absent and this is justified using rigorous wave theory.

### Answered by Thiyagarajan K | 1st Mar, 2020, 12:16: PM

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