Answer: Every point of a given wavefront acts as a source of secondary wavelets; the forward envelope (common tangent surface) of these wavelets after time \(t\) is the new wavefront.
[Diagram: point source S; an arc AB (primary spherical wavefront of radius \(r\)); small circles of radius \(ct\) drawn from points on AB; a larger concentric arc A₁B₁ touching all the small circles in the forward direction (new wavefront); a dotted arc A₂B₂ touching them on the back side (backward wavefront, not present).]
- Let S be a point source of light in a homogeneous isotropic medium. At some instant, the light has reached the spherical surface AB of radius \(r\) centred at S; AB is the primary wavefront.
- According to Huygens' principle, every point on AB becomes a source of secondary wavelets, which travel with the speed \(c\) of light in the medium.
- To find the wavefront after time \(t\), draw spheres of radius \(ct\) with points on AB as centres.
- The surface A₁B₁ that touches all these secondary wavelets in the forward direction is a sphere of radius \(r + ct\) centred at S. This is the new (secondary) wavefront.
- The backward envelope A₂B₂ is ignored, since there is no backward flow of energy (no backward wave).
