Let’s now see how these ideas about interference apply to light waves. FIGURE 33.4a shows an experiment in which a laser beam is aimed at an opaque screen containing two long, narrow slits that are very close together. This pair of slits is called a double slit, and in a typical experiment they are $\approx 0.1\text{mm}$ wide and spaced $\approx 0.5\text{mm}$ apart. We will assume that the laser beam illuminates both slits equally and that any light passing through the slits impinges on a viewing screen. This is the essence of Young’s experiment of 1801, although he used sunlight rather than a laser.

FIGURE 33.4 A double-slit interference experiment.

What should we expect to see on the screen? FIGURE 33.4b is a view from above the experiment, looking down on the top ends of the slits and the top edge of the viewing screen. Because the slits are very narrow, light spreads out behind each slit exactly as it did in Figure 33.2. These two spreading waves overlap and interfere with each other, just as if they were sound waves emitted by two loudspeakers. Notice how the antinodal lines of constructive interference are just like those in Figure 33.3.

The image in Figure 33.4b shows how the screen looks. As expected, the light is intense at points where an antinodal line intersects the screen. There is no light at all at points where a nodal line intersects the screen. These alternating bright and dark bands of light, due to constructive and destructive interference, are called interference fringes. The fringes are numbered $m=0\text{,}1\text{,}2\text{,}3\dots$, going outward from the center. The brightest fringe, at the midpoint of the viewing screen, with $m=0$, is called the central maximum.