The Huygens’ Principle
In 1678, Haans Christian Huygens’ proposed a theory that 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. 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 itself the source of spherical wavelets. 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 carried energy as it travelled.
Huygens’s 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).
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.
The 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).
However, the 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.
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.