Huygens Principle: Definition, Principle and Explanation

Huygens Principle is an interesting topic of Physics that one should study. When you open the window of your room, the light enters through the window and spreads throughout the room. Do you know the reason behind it on how or why does it happen This is because light has got some kind of wave nature, which when enters in your room, spreads in all the directions. This phenomenon is known as Huygens Principle. To understand it in a better way, let us study the Huygens Principle. 

Key Takeaways: Huygens Principle, Light, phenomenon, Wave nature, Wavefront, Wavelets, Waveform

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Huygens Principle

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If we consider a point source, the nature of the wavefront will be a spherical one and it will emit its new wavefront. The Huygens Principle states that each point of the wavefront is the source of the secondary wavelets, it spreads out in all directions with the speed of a wave. As per Huygens’ principle, as the wavefronts will be in the forward direction, all the points on the wavefront are going to become a secondary source. All the secondary sources emit wavelets and tangent, drawn to all the wavelets is the new position of the waveform. 

Huygens Principle

The video below explains this:

Huygens Principle Detailed Video Explanation:

Let’s take an example, suppose you are standing on a mountain and you throw a stone into the water from a height. What will you observe? You will observe that the stone strikes the surface of the water and waves are seen surrounding that point. Simultaneously, every point on the surface of water starts oscillating. The waves spread across all directions. Earlier, the water was at rest. But at the instance, when we throw the stone in the water, within a fraction of second the disturbance spreads across all directions. There are ripples formed within the water. The ripples form the concentric revolving around the disturbance. These ripples are nothing but the wavefront. The wavefronts gradually spread throughout. So at every point, we have a wave beginning. The first wavefront is made and again from the first wavefront, a secondary waveform is made and so on. The disturbance doesn't last for an extended time. It fades gradually because of more formation of waveforms.

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Clarification

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Huygens principle expresses that each point on the wavefront could likewise be viewed as a wellspring of secondary circular wavelets which spreads out in the forward direction at the speed of light. The new wavefront is the digressive surface to all or any of these secondary wavelets. Hence, it's a mathematical technique to search out the frequency. 

Huygens proposed Huygen’s principle in 1678 which explains about daylight and its qualities. You'll be familiar with the rectilinear hypothesis of daylight that indicates that light goes along straight line. Huygen’s principle is one among the vital techniques for considering different optical wonders. The principle is a technique for the study of applied issues of wave engendering both the inside of the far-field limit and the in-field diffraction and furthermore reflection.
It expresses that:
"Each point on a wavefront is in itself the source of round wavelets which spreads out forward at the speed of light. The amount of those round wavelets makes the wavefront". 

This hypothesis didn't clarify why refraction happened. Also, it couldn't clarify how light conveys energy while it travels in the space.

Huygen’s Principle, additionally known as the Huygens–Fresnel Principle. It has following features of wave propogation:

1. Every secondary source forms its own wavelets which are more similiar to the primary source.
2. At any random point, the tangents which are in the forward direction forms a new wavefront.
3. A wavefront is the total of all the spherical wavelets.

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Primary and Secondary Sources

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Huygens expressed that light might be a wave that spreads through space fundamentally, the same as waves in water or sound in air. Thus, light opens up like waves and heading in all direction from a source. The locus of focuses that voyaged far during an immovable span is called a wavefront. Accordingly, from some degree of wellspring of daylight, the locus of focuses that light has gone during a rigid timeframe might be a circle.

When a primary wavefront is created then from every primary wavefront, a secondary wavefront is also created. At the same time, every point on that wavefront will now act as a secondary source of light from which more wavefronts are generated. Like this, the light waves spread out in the space by creating secondary sources and wavefronts.

Primary and Secondary Sources

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Huygens Principle and Diffraction

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At the point when light goes through an aperture, each point on the light wave inside the aperture is a source. It makes a roundabout wave that spreads outward from the aperture. The aperture in this way makes a source of wave that spreads out in the form of spherical waves. The focal point of the wavefront has more noteworthy force while the sides have a lesser power. This explains diffraction more clearly and explains why an ideal picture of the aperture on a screen isn't made. One of the common daily life example of this phenomenon is- If you are in a room which has only one door and somebody from another room shouts. The sound from that person will spreads out in all direction but the sound will comes to you from the door only and rest of the sound will be absorbed by the walls.

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Advantages and Disadvantages of Huygens Principle

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Advantages:

• Huygen’s concept proved the reflection and refraction of sunshine.
• The concepts like diffraction of sunshine and also interference of sunshine were proved by Huygens.

Disadvantages:

• Concepts like emission of sunshine, absorption of sunshine and polarization of sunshine weren't explained by Huygens’ principle.
• Huygen's principle did not explain the photoelectric effect.
• A serious drawback is that the idea proposes an all-pervading medium required to propagate light called luminiferous ether. This was proved to be false within the 20th century.

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Things to Remember

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• The Huygens Principle states that each point of the wavefront is the source of the secondary wavelets, it spreads out in all directions with the speed of a wave.
• Huygens principle expresses that each point on the wavefront could likewise be viewed as a wellspring of secondary circular wavelets which spreads out in the forward direction at the speed of light.
• Huygens proposed Huygen’s principle in 1678 which explains about daylight and its qualities.
• Huygens expressed that light might be a wave that spreads through space fundamentally, the same as waves in water or sound in air.
• Huygen’s concept proved the reflection and refraction of sunshine. 
• Huygen's principle did not explain the photoelectric effect.
• A serious drawback is that the idea proposes an all-pervading medium required to propagate light called luminiferous ether. This was proved to be false within the 20th century.

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Previous Year Questions  

1. When hydrogen atom is in its first excited level, its radius, is….
2. When an electron does transition from n=4 to n=2 , then emitted line spectrum will be….​..
3. α -particle consists of….
4. An electron of a stationary hydrogen atom passes from the fifth energy level….
5. Complete the equation for the following fission process…. [NEET 1998]
6. using non-relativistic approach, the speed of electron in this orbit will be…. [NEET 2015]
7. When the glancing angle of incidence of light on a material is...[COMEDK UGET 2004]
8. Two waves having intensity ratio 25 : 4 produce interference. The ratio of maximum to minima intensity is...[COMEDK UGET 2004]
9. In Young's double slit experiment,1^st dark fringe occurs directly...[COMEDK UGET 2009]
10. In the diffraction pattern due to a single slit linear width of the central max...[COMEDK UGET 2007]
11. In Newton ring experiment, monochromatic light is replaced by white light...[COMEDK UGET 2008]
12. In diffraction through a single slit experiment, slit width is halved...[COMEDK UGET 2004]
13. Which of the following is false for interference of light?...[JKCET 2012]
14. Which of the following is true for the minimum angular separation of two stars...[JKCET 2012]
15. Values for Brewster's angle can be...[JKCET 2015]
16. Unpolarized light falls on two polarizing sheets placed one on top of other….[JKCET 2013]
17. In Young's double slit experiment using monochromatic light of wavelength   \(\lambda\) ….[AMUEEE 2018]
18. In a single-slit diffraction experiment, the width of the slit is reduced by...[JKCET 2012]
19. the wavelength of light illuminating the slits is….[JKCET 2013]
20. Colours in thin films are due to….[JKCET 2008]

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