S is a monochromatic source of light. It is placed on the focal plane of a converging lens so that a plane wavefront is incident on a narrow slit. Another converging lens focuses light from different portions of the slit on the observation screen.
The salient features of the actual diffraction pattern produced by a single vertical slit from a point source are:
According to Huygens’ wave theory, plane wavefronts are incident on the barrier containing the slit. As these wavefronts fall on the barrier, only that part of the wavefront passes through the slit which is incident on it. This part of the wavefront continues to propagate to the right of the barrier. However, the shape of the wavefront does not remain plane beyond the slit.