Discovery 3.1 - The Wave Nature of Radiation

The Wave Nature of Radiation

Until the early nineteenth century, debate raged in scientific circles regarding the true nature of light. On the one hand, the particle, or corpuscular, theory, first expounded in detail by Isaac Newton, held that light consisted of tiny particles moving in straight lines at the speed of light. Different colors were presumed to correspond to different particles. On the other hand, the wave theory, championed by the seventeenth-century Dutch astronomer Christian Huygens, viewed light as a wave phenomenon, in which color was determined by frequency, or wavelength. During the first few decades of the nineteenth century, growing experimental evidence that light displayed three key wave properties—diffraction, interference, and polarization—argued strongly in favor of the wave theory.

Diffraction is the deflection, or "bending," of a wave as it passes a corner or moves through a narrow gap. As depicted in the figure below, a sharp-edged hole in a barrier seems at first glance to produce a sharp shadow, as we might expect if radiation were composed of rays or particles moving in perfectly straight lines. Closer inspection, however, reveals that the shadow actually has a "fuzzy" edge, as shown in this photograph at right of the diffraction pattern produced by a small circular opening. We are not normally aware of such effects in everyday life because diffraction is generally very small for visible light. For any wave, the amount of diffraction is proportional to the ratio of the wavelength to the width of the gap. The longer the wavelength and/or the smaller the gap, the greater the angle through which the wave is diffracted. Thus, visible light, with its extremely short wavelengths, shows perceptible diffraction only when passing through very narrow openings. (The effect is much more noticeable for sound waves, however—no one thinks twice about our ability to hear people even when they are around a corner and out of our line of sight.)

Interference is the ability of two or more waves to reinforce or cancel each other. The second figure shows two identical waves moving through the same region of space. In the first part, the waves are positioned so that their crests and troughs exactly coincide. The net effect is that the two wave motions reinforce each other, resulting in a wave of greater amplitude. This is known as constructive interference. In the second part of the figure, the two waves exactly cancel, so no net motion remains. This is destructive interference. As with diffraction, interference between waves of visible light is not noticeable in everyday experience; however, today it is easily measured in the laboratory (as shown in the diagram).

Finally, the phenomenon known as polarization of light is also readily understood in terms of the description of electromagnetic waves presented in the text. Normally, light waves are randomly oriented—the electric field in Figure 3.7 may vibrate in any direction perpendicular to the direction of wave motion—and we say the radiation is unpolarized. Most natural objects emit unpolarized radiation. Under some circumstances, however, the electric fields can become aligned—all vibrating in the same plane as the radiation moves through space, and the radiation is said to be polarized. On Earth we can produce polarized light by passing unpolarized light through a Polaroid filter, which has specially aligned elongated molecules that allow the passage of only those waves having electric fields oriented in some specific direction. Reflected light is often polarized, which is why sunglasses constructed with suitably oriented Polaroid filters can be effective in blocking glare.

Diffraction and interference play critical roles in many areas of observational astronomy, including telescope design (Chapter 5). The polarization of starlight provides astronomers with an important technique for probing the properties of interstellar gas (Chapter 18). All three phenomena are predicted by the wave theory of light. The particle theory did not predict them; in fact it predicted that they should not occur. Until the early 1800s, the technology was inadequate to resolve the issue. However, by 1830 experimenters had reported unequivocal measurement of each, convincing most scientists that the wave theory was the proper description of electromagnetic radiation. It would be almost a century before the particle description of radiation would resurface, but in radically different form, as we will see in Chapter 4.

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