# The Huygens' Principle

Huygens principle proposed by Christiaan Huygens In 1678 revolutionised our understanding of light and its characteristics. You may be familiar with the rectilinear light theory that suggests light travels along straight paths. Huygens principle is one of the key methods for studying various optical phenomena. In this article, we will learn the Huygens principle and its advantages and disadvantages in detail.

## Defining the Huygens Principle

Huygens principle is a method of analysis applied to wave propagation problems both in the far-field limit and near-field diffraction and reflection. It states that:

“Every point on a wavefront is in itself the source of spherical wavelets which spread out in the forward direction at the speed of light. The sum of these spherical wavelets forms the wavefront”.

However, this theory did not explain why refraction occurred in the first place. Secondly, it could not explain how light carries energy as it travels.

Huygens Principle, also known as the Huygens–Fresnel principle, highlights the following wave propagation behaviour:

• Secondary sources form wavelets similar to the primary source.
• At any given time, the common tangent on the wavelets in the forward direction gives the new wavefront.
• The wavefront is the sum of the spherical wavelets.

### Primary and Secondary Sources

Huygens stated that light is a wave propagating through space like ripples in water or sound in air. Hence, light spreads out like a wave in all directions from a source. The locus of points that travelled some distance during a fixed time interval is called a wavefront. Thus, from a point source of light, the locus of points that light has travelled during a fixed time period is a sphere (a circle if you consider a 2D source).

After the primary wavefront is created, a secondary wavefront is created from every primary wavefront. Secondly, every point on the wavefront acts as a secondary source of light that emits more wavefronts. This way, a light wave propagates through space by generating secondary sources and wavefronts. The net effect is that the effective wavefront generated is tangential to all the secondary wavefronts generated by the secondary sources, as shown in the figure. The direction of the traverse is always perpendicular to the wavefronts.

Huygens’s theory explains the wave theory of light phenomena of diffraction, interference, reflection, and refraction well, considering it was proposed two centuries ago. (The former two phenomena were not even discovered until the 19th century).

Example

If a stone is thrown into the river, it will create waves around that point in the form of ripples which are circular in nature, called a wavefront.

### Huygens Principle and Diffraction

When light passes through an aperture, every point on the light wave within the aperture can be viewed as a source creating a circular wave that propagates outward from the aperture. The aperture thus creates a new wave source that propagates in the form of a circular wavefront. The centre of the wavefront has greater intensity while the edges have a lesser intensity. This explains the observed diffraction pattern and why a perfect image of the aperture on a screen is not created. A daily life example of this phenomenon is common. If someone in another room calls toward you, the sound seems to be coming from the doorway.

• Huygens concept proved the reflection and refraction of light.
• The concepts like diffraction of light, as well as interference of light, were proved by Huygens.

• Concepts like emission of light, absorption of light and polarisation of light were not explained by Huygens principle.
• Huygens principle failed to explain the photoelectric effect.
• A serious drawback is that the theory proposes an all-pervading medium required to propagate light called luminiferous ether. This was proved to be false in the 20th century.

## Frequently Asked Questions – FAQs

Q1

### Why does the sound bend around the corner of a building while light does not?

The wavelength of the visible light is in the order of 0.5 microns, or 0.0005 mm, due to which light will only diffract through very narrow openings. On the other hand, sound waves have a wavelength of the order 1 metre and diffract very easily. This allows sound waves to bend around the corner.

Q2

### Under what conditions does light behave like a wave and as a particle?

When the light interferes with objects several times greater than its wavelength, the light behaves as a wave. When light is made to interact with objects comparable or small compared to its wavelength, it behaves as a particle.

Q3

### Is the fuzzy edges on your shadow a result of diffraction?

Diffraction bends the path of light waves near sharp edges. Some light is scattered into the region we expect to find dark, blurring the shadow boundary.
Q4

### State Huygens’ Principle?

Huygens’s principle states that every point on a wavefront is the source of spherical wavelets. The sum of these spherical wavelets forms the wavefront.
Q5

### Why is the Huygens Principle important?

Huygens’s principle helps us predict and understand light’s classical wave propagation.
Q6

### Can we apply the Huygens principle to radar waves?

As the Huygens principle is a universal principle relating to wavefront formation, it can be applied to radar waves.
Q7

### Who proposed the Huygens principle?

The physicist Christiaan Huygens proposed the Huygens principle.

Q8

### What are the key conclusions of the Huygens principle?

All points on the wavefront could be taken as the origin of secondary spherical wavelets which travel in the forward direction at light speed. The new wavefront is, in fact, the tangential surface of every one of these secondary wavelets. So, it is also a geometrical method to calculate the wavelength.

Q9

### What are the main drawbacks of the Huygens principle?

The Huygens principle cannot explain why refraction exists in the first place. It also cannot explain how light holds energy as it propagates.

Q10

### What are the advantages of the Huygens principle?

The Huygens principle easily proved the refraction and reflection of light. The concepts such as diffraction of light and interference of light can also be proved by Huygens.

Q11

### What is the refraction of light?

It is a phenomenon through which light beams bend and move from one transparent material to another.

Q12

### Who proposed the wave theory of light?

Christiaan Huygens proposed the wave theory of light.

Q13

### What is the diffraction of light?

Diffraction of light is the bending of light around the edges of a body such that the light rays spread out and illuminate the regions where shadows are typically expected. Generally, it is hard to distinguish diffraction from interference as both happen simultaneously.

Q14

### What is the interference of light?

Light interference happens when two light waves superpose to create new light waves of higher or lower amplitude.

Now, you have an idea of what Huygen’s principle states. If you wish to learn more physics concepts with the help of interactive video lessons, download BYJU’S – The Learning App.

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