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Diffraction
The bending of light waves around the corners of an obstacle or aperture is called diffraction of light.
The phenomenon of diffraction is divided mainly in the following two classes
(a) Fresnel class
(b) Fraunhofer class
S.No  Fresnel Class  Fraunhofer Class 
1  The source is at a finite distance  The source is at infinite distance 
2  No opticals are required.  Opticals are in the form of Collimating lens and focusing lens are required. 
3  Fringes are not sharp and well defined.  Fringes are sharpend well defined. 
Fraunhofer Diffraction at a Single Slit
Linear Width 0f central maximum 2Dλ / a = 2fλ / a
Angular width of central maximum = 2λ / a
where, λ = wavelength of light, a = width of single slit, D = distance of screen from the slit and f = focal length of convex lens.
For Secondary Minima
(a) Path difference = nλ
(b) Linear distance = nDλ / a = nfλ / a
(c) Angular spread = nλ / a
where, n = 1, 2, 3,.,.
For Secondary Maxima
(a) Path difference = (2n + 1) λ/ 2
(b) Linear distance = (2n + 1) Dλ / 2a = (2n + 1) fλ / 2a
(c) Angular spread = (2n + 1) λ / 2
Important Points
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