Double Slit Experiment Revisited

Given our understanding of light as waves, it is possible to give a more accurate representation of the double slit interference pattern given in Fig.10.1.6. The beam of light approaching the two slits from left consists of approximately plane waves, whose crests are shown in Fig.10.2.3.

  

Figure 10.17: Wave description of the two slit experiment

These plane waves diffract at the slits to produce circular wave fronts emerging on the right. These circular wave fronts interfere with each other in the sense that whenever crests meet, they will add (interfere constructively) but whenever a trough meets a crests, the will subtract (interfere destructively). This accounts perfectly for the bright and dark spots observed on the screen.

The presence of such a diffraction/interference pattern is therefore a sure sign of a wave property, as it is very difficult to imagine even qualitatively how such a pattern could result from a beam of particles shone at such a grating. However, we must keep in mind that

Thus, as we shall see shortly, for visible light one needs slits that are about 10^-7 m wide.