and refraction are proved only with the help of wave optics. Though light propagates as a wave, its direction of propagation is still represented as a ray. A good example for wave propagation is the spreading of circular ripples on the surface of still water from a point where a stone is dropped. The molecules (or) particles of water at a point are moving only up and down (oscillate) when a ripple passes through that point.
All these particles on the circular ripple are in the same phase of vibration as they are all at the same distance from the center. The ripple represents a wavefront as shown in Figure . (a). A wavefront is the locus of points which are in the same state (or) phase of vibration.
. Unit wave optics - - - - Unit wave optics . . Huygens’ Principle Huygens principle is basically a geometrical construction which gives the shape of the wavefront at any time if we know its shape at t = .
According to Huygens principle, each point on the wavefront behaves as the source of secondary wavelets spreading out in all directions with the speed of the wave. These are called as secondary wavelets . The envelope to all these wavelets gives the position and shape of the new wavefront at a later time . Thus, Huygens’ principle explains the propagation of a wavefront.
The propagation of a spherical and plane wavefront can be explained using Huygens’ principle. Let, AB be the wavefront at a time, t = . According to Huygens’ principle, every point on AB acts as a source of secondary wavelet which travels with the speed of the wave (speed of light c ). To find the position of the wavefront after a time t , circles of radius equal to ct are drawn with points P , Q, R ...
etc., as centers on AB . The forward envelope (or) the tangent ′ ′ A B of the small circles is the new wavefront at that instant t . The wavefront