Here we have a source s emitting waves having plane wavefronts.
As the wavefronts pass through the slit ab,they are diffracted and are able to reach even those regions behind ab which they would be unable to reach had the rays not bended.
One more thing to be noted here is that the shape of the wavefronts change as they pass through the slit.
The reason for this bending can be explained with the help of huygens principle.
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.
The dimensions of the slit are finite.As a result,applying huygen’s principle we can say that the new wavefront obtained will be something like the surface shown in figure.
When the distance between the slit ab and source of light s as well as between slit ab and the screen is finite, the diffraction is called Fresnal diffraction.
In Fresnal diffraction the waves are either spherical or cylindrical.
If light incident on slit ab is coming from infinite distance, the distance between obstacle a and screen c is infinite, the diffraction is called Fraunhofer diffraction.
In Fraunhofer diffraction the incident waves should have plane wavefronts.
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
20.
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