Huygens’ construction of a spherical wave front: Consider a point source of monochromatic light S in a homogeneous isotropic medium. The light waves travel with the same speed v in all directions. After time f, the wave will reach all the points which are at a distance vt from S. This is spherical wave front XY. Let, A, B, C, …,, be points on this wave front.
Huygens construction of a spherical wave front
To find the new wave front after time T, we draw spheres of radius vT with A, B, C,…, as centres. The envelope or the surface of tangency of these spheres is the surface A’B’C’. This is the new spherical wave front X’Y’. Thus, in an isotropic medium, spherical wave fronts are propagated as concentric spheres.