for the law of reflection, refer to figure . the line ab represents a wave front of the incident light just as ray l surikes the surface. at this instant.the wave at a sends out a huygens wavelet the circular are.centered on a toward d .at the same time.
the wave at b emits a huygens wavelet the circular are centered on b toward c.figure a shows these wavelets after a time interval .after which ray 2 strikes the surface. because both rays 1 and 2 move with the same speed. we must have ad=bc