What is wave front? Explain laws of refraction of light on the basis of Huygens wave theory. Draw necessary ray diagram.

A wavefront is the locus of all points in the field of an optical disturbance having same phase at a given instant. Here we must take into account a different speed of light in the upper and lower media. If the speed of light in vacuum is c, we express the speed in the upper medium by the ratio c / n i , where n i is the refractive index. Similarly, the speed of light in the lower medium is c / n t . The points D, E and F on the incident wavefront arrive at points D, J and I of the plane interface at different times. In the absence of the refracting surface, the wavefront GI is formed at the instant ray DF reaches I. During the progress of ray CF from F to I in time t, however, the ray AD has entered the lower medium, where the speed is different. Thus if the distance DG is v i t a wavelet of radius v t t t t is constructed with center at D. The radius DM can also be expressed as D M = v t t D M = v t D G v i Similarly, a wavelet of radius (ni/nt)JH is drawn centered at J. The new wavefront KI includes point I on the interface and is tangent to the two wavelets at points M and N. The geometric relationship between the angles θ i and θ t formed by the representative incident ray AD and refracted ray DL, is Snell's law, which may be expressed as n i sin θ i = n t sin θ t