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Huygens’ Principle

Huygens principle, also known as the Huygens-Fresnel principle proposed the wave nature of light. The principle establishes a wave view of light rather than a particle view. It allows us to understand and describe the movement of waves. It is a significantly important method for studying various optical phenomena. 

History

The principle was proposed by Christiaan Huygens, a Dutch mathematician, physicist, and astronomer in 1678. It was named after Christiaan Huygens and French physicist Augustin-Jean Fresnel.

Huygens assumed that the light wave travels only in the forward direction for which explanation wasn’t given.

In 1818, Fresnel was able to prove rectilinear propagation of light and diffraction effects by combining his theory of interference and Huygens’ Principle. The assumptions made by Fresnel had no obvious physical foundation but they led to predictions that agreed with many other experimental observations such as the Poisson Spot.

Huygens’ Principle Explained

Huygen’s Principle demonstrated on plane and spherical wavefronts
Huygen’s Principle demonstrated on the plane and spherical wavefronts (Source)

Statement: Every point on a wavefront is in itself the source of a secondary disturbance and the spherical wavelets from these points spread out in the forward direction at the speed of light.

A wavefront is a line or surface in the path of wave motion on which the disturbances at every point have the same phase. In a particular wavefront, at a given moment of time, all particles of the medium are undergoing the same motion.

According to Huygens, a light wave is propagated by the particles in an aether medium. Various particles of the medium, when reached by a wavefront, become secondary sources of light-emitting secondary wavelets. The geometrical envelope of these wavelets at any given later instant represents the new position of the wavefront at that instant. 

Wavefront from a primary source, upon reaching a different medium, creates secondary sources. These secondary sources form their own wavelets that are similar to the wavelets of the primary source. A common tangent to all the wavelets created by secondary sources gives the new wavefront. The wavefront is the sum of the spherical wavelets.

Huygens’ Principle Examples

An example of the operation of the Huygens’ principle is when an open doorway connects two rooms and a sound is produced in a remote corner of one of the rooms. A person in the other room will hear the sound as if it originated at the doorway.

Another example is if a stone is thrown into the river it will create waves or ripples around that point. These waves look like circular rings and are called wavefront waves. Gradually, these wavefronts disperse in all directions. These ripples are carried forward in all directions by the secondary sources and the secondary sources of these sources and so on.

Applications

  • It served as a fundamental explanation of the wave nature of the light interference.
  • Diffraction of light can be easily explained using Huygen’s Principle. When light passes through an aperture, every point of the light wave within the aperture can be viewed as creating a circular wave that propagates outward from the aperture, which is treated as creating a new wave source. The centre of the wavefront has greater intensity, with a fading of intensity as the edges are approached.
  • The laws of reflection and refraction can both be derived from Huygens’ principle. Points along the wavefront are treated as sources along the surface of the refractive medium, at which point the overall wave bends based upon the new medium.
  • It is a useful technique to determine how and where the waves propagate.
  • It helps in explaining the linear and spherical wave propagation.

Limitations

  • Huygens’ principle failed to provide reasons for wave propagation in backward directions.
  • The principle failed to explain the rectilinear propagation of light.
  • It also failed to prove the concept of polarisation of light, emission of light, absorption of light, and the photoelectric effect.

FAQs

What are wavefronts?

The locus of all the points which are in the same phase is known as a wavefront.

What is meant by diffraction? 

It is the bending of light as light passes through an aperture or around obstacles.

What is an aether medium?

Until the 19th century, it was widely believed that light(and other EM waves) passed through a medium known as the aether medium. It was assumed to be transparent, weightless, undetectable, but permeating all space.

When does light behave like a wave and a particle?

Light behaves as a wave or a particle depending upon the circumstances and the nature of the respective physical settings.
Light acts as a wave when it interferes with obstacles several times greater than its wavelength, whereas it acts as a particle when it interferes with objects that are of small or comparable wavelengths.

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