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Huygens’ Principle – Class 12 Physics (CBSE)

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Nayan Jha

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Huygens’ Principle

1. Statement of Huygens’ Principle

Statement 1: Every point on a wave-front acts as a source of secondary wavelets that spread out in all directions with the speed of light in that medium.

Statement 2: The forward envelope (common tangent surface) of these secondary wavelets at any later instant gives the new wave-front at that instant.

In another words we can state –

Huygens’ principle states that every point on a wavefront acts as a source of secondary wavelets, which spread out in the forward direction with the speed of light in that medium. The new wavefront at any later time is the forward envelope of these secondary wavelets.

2. Explanation of Huygens’ Principle

Huygens Principle Explains Wave Behavior | Dr. Krishnakant Prasad posted on the topic | LinkedIn

Consider a wavefront (AB) at a particular instant.

  1. Every point on the wavefront AB acts as a source of secondary wavelets.
  2. These secondary wavelets travel forward through the medium with the speed of light.
  3. After a time t, each wavelet travels a distance ct, where c is the speed of light in the medium.
  4. A common tangent surface drawn to these wavelets gives the new wavefront A’B’.

Thus, Huygens’ principle helps us determine the position of a wavefront at any later instant.

3. Uses of Huygens’ Principle in Wave Optics

Huygens’ principle is used to explain and derive:

  1. Reflection of light: It helps derive the laws of reflection.
  2. Refraction of light: It helps derive Snell’s law of refraction.
  3. Wave propagation: It explains how a wavefront advances through a medium.
  4. Interference of light: The wave nature of light and the superposition of waves help explain interference.
  5. Diffraction of light: It helps explain the bending and spreading of light around obstacles and through narrow apertures.

Important result for examinations

Using Huygens’ principle, we can prove the laws of reflection:

  • Angle of incidence = Angle of reflection, i=r.
  • The incident ray, reflected ray and normal at the point of incidence lie in the same plane.

We can also derive Snell’s law for refraction:

sin i / sin r=v1/v2

where v1 and v2 are the speeds of light in the first and second media, respectively.

Conclusion: Huygens’ principle is a fundamental concept of wave optics used to understand the propagation of light and derive the laws of reflection and refraction.

Why two independent sources of light can’t be coherent? To know the answer click here-

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