acoustical waves. For example, diffraction is readily demonstrated in the laboratory by mechanically gener-ating waves of constant frequency in a tank of water and observing the wave crests before and after they pass through a rectangular opening or slit. When the slit is wide relative to the wavelength (Figure 6-7a), diffrac¬tion is slight and difficult to detect. On the other hand, when the wavelength and the slit opening are of the same order of magnitude, as in Figure 6-7b, diffraction becomes pronounced. Here, the slit behaves as a new source from which waves radiate in a series of nearly 180-deg arcs. Thus, the direction of the wave front ap¬pears to bend as a consequence of passing the two edges of the slit.
Diffraction is a consequence of interference. This relationship is most easily understood by considering an
experiment, performed first by Thomas Young in 1800, in which the wave nature of light was unambiguously demonstrated. As shown in Figure 6-8a, a parallel beam of light is allowed to pass through a narrow slit A (or in Young’s experiment, a pinhole) whereupon it is dif¬fracted and illuminates more or less equally two closely spaced slits or pinholes B and C; the radiation emerging from these slits is then observed on the screen lying in a plane XY. If the radiation is monochromatic, a series of dark and light images perpendicular to the plane of the page is observed.
Figure 6-8b is a plot of the intensities of the bands as a function of distance along the length of the screen. If, as in this diagram, the slit widths approach the wave¬length of radiation, the band intensities decrease only gradually with increasing distances from the central band. With wider slits, the decrease is much more pro-nounced.
In Figure 6-8a, the appearance of the central band E, which lies in the shadow of the opaque material separat¬ing the two slits, is readily explained by noting that the paths from B to E and c to E are identical. Thus, con¬structive interference of the diffracted rays from the two