We opened this chapter with a photograph (Figure 33.1a) of a water wave passing through a hole in a barrier, then spreading out on the other side. You then saw an image (Figure 33.2) showing that light, after passing through a very narrow slit, also spreads out on the other side. This is called diffraction.

FIGURE 33.11 shows the experimental arrangement for observing the diffraction of light through a narrow slit of width $a$. Diffraction through a tall, narrow slit is known as single-slit diffraction. A viewing screen is placed distance $L$ behind the slit, and we will assume that $L\gg a$. The light pattern on the viewing screen consists of a central maximum flanked by a series of weaker secondary maxima and dark fringes. Notice that the central maximum is significantly broader than the secondary maxima. It is also significantly brighter than the secondary maxima, although that is hard to tell here because this image has been overexposed to make the secondary maxima show up better.

FIGURE 33.11 A single-slit diffraction experiment.