Single Slit Interference & Diffraction Pattern | Physics for ACT PDF Download

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

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

• A soap bubble or oil film on water appears coloured in white light due to interference of light reflected from upper and lower surfaces of soap bubble or oil film.
• In interference fringe pattern all bright and dark fringes are of same width,
• In diffraction fringe pattern central bright fringe is brightest and widest. and I remaining secondary maxima's are of gradually decreasing intensities.
• The difference between interference and diffraction is that the interference is the superposition between the wavelets coming from two coherent sources while the diffraction is the superposition between the wavelets coming from the single wavefront