A Detailed Understanding Of Huygen’s Principle

Also known as Huygen-Fresnel Principle, Huygen’s Principle was introduced by Dutch mathematician, physicist, and astronomer Christiaan Huygen in 1690. The theory has been of significance in understanding a variety of optical phenomena. 

According to Huygen’s Principle, each point of a wavefront can be considered a secondary source of light, and these points emit spherical wavelets of light. The waves emitted are said to be semicircular in nature, and the new wavefront can act as a tangent to the secondary wavelets. Huygen’s Principle also states that these secondary wavelets propagate with the velocity of light in the same medium.

Huygen’s Principle could also help derive the laws of reflection and refraction and could explain the phenomenon of interference in light. 

Before we get into the details, there are some concepts that must be understood in order to understand Huygen’s Principle. Let us start with the most significant one, that is, wavefront.

Wavefront

A wavefront can be described as any surface, real or imaginary, representing the optical wave vibrating in unison. These can be represented either on the crest or the trough of the same wave. For example, when you throw a stone in the river, it creates a ripple effect. You can see circular rings being formed on the water. It is in these circular rings that the waves vibrate in sync with each other. The particles inside the wave move in the same phase. These waves of constant phase are known as wavefronts.

Plane wavefronts have rays parallel to each other in constant amplitude, whereas spherical wavefronts are formed from a spherical point source.

Wave Theory Of Light

Alongside Huygen’s Principle, the wave theory of light must also be understood. Huygens proposed that light waves were longitudinal. Later, it was found that light waves were transverse in nature, meaning that particles travelled perpendicular to the direction of the light. 

This came to be known as the Wave Theory of Light. Wavelength and frequency were two important concepts of light waves. Wavelength is defined as the distance between two waves. Frequency is the number of peaks that travel beyond a point in one second.

The Wave Theory of Light was used to explain many optical phenomena, but could not explain the photoelectric effect of light.

Laws Of Reflection and Refraction

Huygen’s Principle has been significant in explaining various optical phenomena and helping in deriving the laws of reflection and refraction using the principle.

Huygen’s Principle also explains light’s nature of interference. It states that the diffusion of waves does not take place while propagating. For example, a person standing at a distance can see a lightbulb being turned on, or the ability to see clear pictures of stars from such a long distance can be attributed to Huygen’s Principle.

Huygen-Fresnel Principle

Huygen’s Principle can be used to explain not only the phenomena of lightwaves, but also sound waves and electromagnetic waves. But Huygen’s Principle could not explain the diffraction or polarisation of light.

It was Augustin Fresnel who later modified Huygen’s Principle and explained the diffraction phenomena. It was Fresnel himself who mentioned that light waves were transverse. He used the wave theory of light to explain the rectilinear propagation of light. 

Gustav Kirchhoff later arrived at the Huygen-Fresnel Principle through the Maxwell Equation. Huygen-Fresnel Principle states that every point of a wavefront is a source for secondary wavelets, and the resulting amplitude of these wavelets is the sum of all the individual waves.

Conclusion

To summarise, Huygen’s Principle was proposed by Christiaan Huygen in 1690. It is a theory that states that each point of a wavefront is a source for secondary wavelets. It can also be used to derive laws of reflection and refraction. Huygen’s Principle can also be applied to different types of waves, especially sound waves. The theory was later modified by Augustin Fresnel to become the Huygen-Fresnel Principle, which could also explain the diffraction of light.