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.
However this theory did not explain why refraction occurred in the first place. Secondly, it could not explain how light carried energy as it traveled.
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 all directions from a source. The locus of points that traveled some distance during a fixed time interval is called a wave front. Thus, from a point source of light the locus of points that light has traveled 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 wave fronts generated by the secondary sources as shown in the figure. In this way ,a light wave propagates through space by generating secondary sources and wave fronts. The direction of traverse is always perpendicular to the wave fronts.
The Huygens’s theory explains the 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. For detailed explanation on this topic , download Byju’s – learning app.