Huygen's principle of secondary wave fronts for spherical and plane wave fronts Diffraction
A point source of light emits spherical waves. After any given interval of time t all the particles of the medium on the surface of the spherical wavefront(locus of all particles in the medium which are in the same state of vibration) will be vibrating in phase. This is known as a spherical "primary wavefront".
According to Huygens principle, all points on the primary wavefront are sources of secondary disturbance. These secondary waves travel through space with the same velocity as the original wave and the envelope of all the secondary wavelets after any given interval of time gives rise to the "secondary wavefront" assuming that waves from a source of light propagate outwards. All points concentric to the source will have crests (or troughs) at the same distance.
Huygen's wave theory explains both reflection and refraction. According to wave theory, a light source gives off light waves which spread out in all directions. If any of the waves strike a mirror, they are reflected according to the angles from which they arrive. Reflection turns each wave back to front – this is why the image seen is reversed.