Q.Distinguish between the phenomenon of interference and diffraction of light.

Interference results from superposition of waves from two coherent sources; diffraction results from superposition of secondary wavelets from different parts of the same wavefront.

Interference:

1. Occurs due to superposition of two separate coherent wave trains, from two distinct (coherent) sources.
2. All bright (or dark) fringes are generally of equal width and equal intensity (uniform fringe pattern).
3. There is a good, uniform contrast: intensity at minima can fall to (near) zero.
4. Fringe width depends on the separation between the two sources and the source-to-screen distance ($\beta=\lambda D/d$).

Diffraction:

1. Occurs due to superposition of secondary wavelets originating from different points of the same wavefront (bending of light around obstacles/through apertures).
2. The central maximum is much brighter and wider than the secondary maxima, and the secondary maxima/minima are of progressively decreasing intensity, unequally spaced. …