Huygens’s Principle describes the propagation of the wave fronts of light by the formation and superposition of secondary wavelets. This principle is critical in the study of Wave Optics.
In the image above, we can see that there is a wave front made up of the superposition of infinite wavelets that emanate from the previous wave front. If the wave front starts as a line, it will continue as a line (diagram a). If it starts as a circle, then it will continue as a circle (diagram b).