Using Huygens's construction of secondary wavelets explain how a diffraction pattern is obtained on a screen due to a narrow slit on which a monochromatic beam of light is incident normally.
Consider a parallel beam of light from a lens falling on a slit AB. As diffraction occurs, the pattern is focused on the screen with the help of lens L2. We will obtain a diffraction pattern that is a central maximum at the centre O, flanked by a number of dark and bright fringes called secondary maxima and minima.
Each point on the plane wave front AB sends out secondary wavelets in all directions. The waves from points equidistant from the centre C, lying on the upper and lower half, reach point O with zero path difference and, hence, reinforce each other producing maximum intensity at point O.