Consider a point source of monochromatic light S in a homogeneous medium. The light waves travel with the same speed(v) in all the directions. After the time 't', the waves will reach all the points which are at a distance vt from S. This is the spherical wavefront XY. Let A, B, C be points on the wavefront. To find the new wavefront after a time Δt, we draw spheres of radius vΔt with A, B and C as centres. The tangents drawn to these spheres is the new spherical wavefront.