(a) State Huygens principle. How did Huygens justify the absence of the backwave on a spherical wavefront ? OR (b) In a single-slit diffraction experiment, light of wavelength $\lambda$ illuminates the slit of width 'a'. The diffraction pattern is observed on a screen kept at a distance D from the slits. (i) Depict variation of intensity in the fringe pattern with the angular position of the fringes. (ii) How is the linear width of central maximum affected when separation between the slit and the screen is decreased ?