Q.Distinguish between interference and diffraction of light. A double slit arrangement produces interference fringes for sodium light of wavelength 589 nm, that are 0.20 degree apart. What is the angular fringe separation if the entire arrangement is immersed in water? (R.I. of water = 1.33)

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. …