What is the Huygens principle formula and how does it explain the propagation of light waves?

The Huygens principle states that each point on a wavefront acts as a source of secondary wavelets that spread out in all directions. The formula for the Huygens principle is: r d/D, where r is the distance between wavelets, is the wavelength of light, d is the distance between the wavefront and the point of interest, and D is the distance from the wavefront to the screen. This principle helps explain how light waves propagate by showing how each point on a wavefront generates new wavelets that combine to form the overall wave pattern.