(a) Describe any two characteristic feature which distinguish interference and diffraction phenomena. Derive the expression for the intensity at a point of the interference pattern in Young's double slit experiment.
(b) In the diffration due to single slit experiment, the aperture of the slit is 3 mm. If monochromatic light of wavelength 620 nm in incident normally on the slit, calculate the separation in between the first order minima and the `3^("rd")` order maxima on one side of the screen. The distance between the slit and the screen is 1.5 m.

Explanation

Given `a = 3 mm = 3 xx 10^(-3)m, lambda = 620 nm = 620 xx 10^(-9) m, D = 1.5 m`
Distance of first order minima from centre of the central maxima = `x_(D1) = lambda D//a`
Distance of third order maxima from centre of the central maxima `x_(D3) = 7 lambdaD//2a`
Distnace between first order minima and third order maxima = `x_(D3) - x_(D1)`
`= (7D lambda)/(2a) - (lambda D)/(a) = (5D lambda)/(2a) = (5 xx 620 xx 10^(-9) xx 1.5)/(2 xx 3 xx 10^(-3))`
`= 7.75 xx 10^(-4) m`.