Wavefront is defined as the locus of all the points in space that reach a particular distance by a propagating wave at the same instant.Huygen s principle:i) Each point of the wavefront is the source of a secondary disturbance and the wavelets emanating from these points spread out in all directions. These travel with the same velocity as that of the original wavefront.ii) The shape and position of the wave-front, after time t , is given by the tangential envelope to the secondary wavelets.A plane wavefront is incident on a thin convex lens:                                                    ORi) Reflection and refraction arise through interaction of incident light with atomic constituents of matter which vibrate with the same frequency as that of the incident light. Hence, frequency remains unchanged.ii) No, energy carried by a wave depends on the amplitude of the wave, not on the speed of wave propagation.iii) For a given frequency, intensity of light in the photon picture is determined by the number of photon incident normally on crossing unit area per unit time.