# The Huygens' Principle

View Notes

## The Huygensâ€™ Principle and the Principle of a Wave Front

The Huygensâ€™ principle which is present in optics is said to be a statement that is - all points which are of a wave front of light in a vacuum or transparent medium may be regarded as new sources of wavelets. Here, we are going to discover more information about the topic. The principle was proposed by the Dutch mathematician and physicist as well as an astronomer- Christiaan Huygens that too in 1690. It is said to be a very powerful method for studying various phenomena of optics.

### Huygens Principle

A tangent surface to the wavelets usually constitutes the new wave front and is known as the envelope of the wavelets. It can be said that if a medium is homogeneous and has the same properties throughout or is isotropic and it allows light to travel with the same speed regardless of its direction of propagation. This implies that the envelope of the three-dimensional point source will be spherical or otherwise as is the case with many crystals we will see that the envelope will be ellipsoidal in shape. we may be familiar with the rectilinear theory of light that generally purports that light travels along straight paths. The principle of Huygens is one of the key methods for studying various optical phenomena. The principle is a method of applied analysis of problems in wave propagation both in the far-field limit and in near-field diffraction and also reflection. It states the following:

• The sum of these spherical wavelets generally forms the wavefront. However, we can say that this theory did not explain why refraction occurred in the first place.Â

• Secondly, we can also notice that it could not explain how light carries energy as it travels.

The principle of Huygens Principle also called as the Huygensâ€“Fresnel principle highlights the following wave propagation behaviour:

• We can say that secondary sources form their own wavelets that are similar to that of the primary source.

• We will see that at any given point in time the common tangent on the wavelets in the forward direction generally gives the new wavefront.

• The wavefront which we have learnt here is the sum of the spherical wavelets.

### SourcesÂ

Huygens earlier stated that light is a wave that propagates through space much like ripples which are in water or we can say the air in sound. Hence, light usually spreads out like a wave along with all directions from a source. The points locus that travelled some distance that too during a fixed time interval is known as a wavefront. Thus we can notice that from a point source of light the locus of points that light has travelled during a fixed period of time is a sphere.

After the completion of the primary wavefront creation, a secondary wavefront is created too from every primary wavefront. Secondly, we will see that every point on the wavefront acts as a secondary source of light that emits more wavefronts. So we can say that in this way a light wave propagates through space by generating secondary sources and wavefronts. The traverse direction is always said to be perpendicular to the wavefronts.

The theory of Huygensâ€™ explains to us that the wave theory of light phenomena of diffraction and interference, and reflection and refraction well, considering it was proposed two centuries ago. The former two phenomena which we have mentioned were not even discovered until the 19th century.

### PrincipleÂ

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

• The concept of Huygensâ€™ proved the reflection and refraction of light.

• The concepts t like diffraction of light as well as interference of light is proved by Huygensâ€™ principle.

• The whole concepts like emission of light and absorption of light and polarization of light were not explained by the principles of Huygensâ€™.

• The principles of Huygensâ€™ generally failed to explain the photoelectric effect.

• A serious drawback is that the theory proposes an all-pervading medium required to propagate light known as the luminiferous ether.Â

1. Explain the wavefront and Huygensâ€™ principle.

The principle of Huygens's states that every point on a wavefront is a source of wavelets. These wavelets which we know already by reading the article are spread out in the forward direction at the same speed as the source wave. The new wavefront is a tangent line to all of the wavelets.

2. Explain why the Huygensâ€™ principle is important.

The principle of the Huygens usually works for all types of waves, that is including water waves and the sound waves and light waves. It is said to be useful not only in describing how light waves propagate but also in explaining the reflection and refraction laws.

3. What do you mean by Huygensâ€™ principle?

A principle in physics: which states that every point of an advancing wave front is a new centre of disturbance from which the independent wavelets emanate independently, whose envelope constitutes a new wavefront at each successive stage of the process is Huygensâ€™ principle.

4. Is Huygens' principle correct?

We can say that the Huygensâ€™ principle is not actually correct in optics.Â  This is said to be a consequence of the fact that the equation of the wave in optics is second order in time.