Scientists of the 17th and 18th centuries might have reached a different conclusion about the nature of light had they seen the experiment depicted in FIGURE 33.2. Here light of a single wavelength (or color) passes through a narrow slit that is only 0.1 mm wide, about twice the width of a human hair. The image shows how the light appears on a viewing screen 2 m behind the slit. If light travels in straight lines, as Newton thought, we should see a narrow strip of light, about 0.1 mm wide, with dark shadows on either side. Instead, we see a band of light extending over about 2.5 cm, a distance much wider than the aperture, with dimmer patches of light extending even farther on either side.

FIGURE 33.2 Light, just like a water wave, does spread out behind an opening if the opening is sufficiently narrow.

If you compare Figure 33.2 to the water wave of Figure 33.1a, you see that the light is spreading out behind the opening. The light is exhibiting diffraction, the sure sign of waviness. We will look at diffraction in more detail later in the chapter. For now, we merely need the observation that light does, indeed, spread out behind an opening that is sufficiently narrow. Light is acting as a wave, so we should—as Thomas Young did—be able to observe the interference of light waves.