Diffraction at a Single Slit

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:

  • A horizontal streak of light along a line normal to the length of the slit.
  • The horizontal pattern is a series of bright spots.
  • The spot at the centre is the brightest. On either side of this spot, a few more symmetrically situated bright spots of diminishing intensity are observed. The central spot is called principal maxima and other spots are called secondary maxima.
  • The width of the central spot is twice the width of other spots.

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.