Describe any two characteristic feature which distiguish 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.