The Huygens' Principle And The Principle Of A Wave Front - Physics

Defining the Huygens Principle

Huygens principle proposed by Christiaan Huygens In 1678 revolutionized our understanding of light and its characteristics. You may be familiar with the rectilinear theory of light that purports that light travels along straight paths. Huygens principle is one of the key methods for studying various optical phenomena. The principle is a method of analysis applied to problems of wave propagation both in the far-field limit and in near-field diffraction and also 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 their own wavelets that are similar to the of the primary source.
• At any given point in 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 that propagates through space much like ripples in water or sound in air. Hence, light spread out like a wave along with 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. 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. In this way, a light wave propagates through space by generating secondary sources and wavefronts. The direction of the traverse is always perpendicular to the wavefronts.

Huygens 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 as 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 tells us 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 towards you, the sound seems to be coming from the doorway.

Advantages and Disadvantages of Huygens Principle

Advantages:

• 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.

Disadvantage:

• Concepts like emission of light, absorption of light and polarization 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.