Huygen’s principle :-each particle lying on any wavefront acts as an independent secondary source and emits from itself secondary spherical waves.After a very small time interval, the surface tangential to all these spherical wavelets,gives the position and shape of the new wavefront.
It will be more clear from the following example of plane wavefronts.
Let p1,p2,p3,…pn be points very close to each other and equidistant from each other on the plane incident wavefront.
We have seen how we can get an interference pattern when there are two slits. We will also get an interference pattern with a single slit provided it’s size is approximately (neither too small nor too large)
To understand single slit diffraction, we must consider each point along the slit (of width a ) to be a point source of light. There will be a path difference between light leaving the top of the slit and the light leaving the middle. This path difference will yield an interference pattern.
Path difference of rays to P from top and bottom edge of slit
L = a sin destructive if L = m , m=1,2,…
Light P (a/2) sin
Single Slit Diffraction Notice that central maximum is twice as wide as secondary maxima Sin = m / W, Destructive Dark Fringes on screen y = L tan L (m W Maxima occur for y= 0 and, y L (m 1/2)( W m=1 m= 1 L