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CHAPTER 10 WAVE OPTICS

Huygens principle

1. Every point on given wave front (called primary wave front) acts as a fresh source of new disturbance called secondary wavelets.

2. The secondary wavelets travel in all the directions with the speed of light in the medium.

3. A surface touching these secondary wavelets tangentially in the forward direction at any instant gives the new (secondary) wave front of that instant

If we wish to determine the shape of the wave front at time = τ, we draw spheres of radius from each point on the spherical wave front where v represents the speed of the wave in the medium.

Common tangent to all these spheres gives the new position of the wave front at t = τ.

Huygens proposed that the amplitude of the secondary wavelets is maximum in the forward direction and zero in the backward direction. Huygens could explain the absence of the back wave using this assumption, but it is not correct. The easiest explanation is that the backward-propagating portions are destructively interfered with by the wavelets from all of the other wave fronts.