Wave optics is the branch of optics that deals with the production, emission and propagation of light, it's nature and the study of phenomenon of interference, diffraction and polarization.
Wave front is the locus of all adjacent points at which the phase of vibration of a physical quantity associated with the wave is the same.
Wave front may be either spherical, cylindrical and plane.
Electromagnetic spectrum is orderly distribution of electromagnetic radiations according to their wavelength or frequency.
Every point on the wavefront acts as a source of disturbance, called wavelets.
i. Each point on a wavefront acts as a secondary source of light. The newly produced waves are called wavelets or secondary waves.
ii. The secondary waves spread out in all direction with the speed of light in a medium.
iii. The new wavefront at any later time is given by the tangential surface in the forward direction of the secondary wavelets at that time.