| Is the sky blue because it reflects the color of the ocean, or is the ocean blue because it reflects the color of the surrounding sky? The answer is the latter, and the reason has to do with the way that light is scattered by air molecules and minute dust particles. By scattering we mean the process by which radiation is absorbed and then reradiated by the material through which it passes.
As sunlight passes through our atmosphere, it is scattered by gas molecules in the air. The British physicist Lord Rayleigh first investigated this phenomenon about a century ago, and today it bears his name—it is known as Rayleigh scattering. The process turns out to be very sensitive to the wavelength of the light involved.
Rayleigh found that blue light is much more easily scattered than red light, in essence because the wavelength of blue light (400 nm) is closer to the size of air molecules than is the wavelength of red light (700 nm). He went on to prove that the amount of scattering is actually inversely proportional to the fourth power of the wavelength, so that blue light is scattered about nine times (that is, [700/400]4) more efficiently than red light. Dust particles also preferentially scatter blue light, but the amount of scattering by dust is just inversely proportional to the wavelength, so the contrast between red and blue scattering is only a factor of (700/400), or 1.75.
Consequently, with the Sun at a reasonably high elevation, the blue component of incoming sunlight will scatter much more than any other color component. Thus, some blue light is removed from the line of sight between us and the |
Sun and may scatter many times in the atmosphere before eventually entering our eyes, as shown in the accompanying figure. Red or yellow light is scattered relatively little and arrives at our eyes predominantly along the line of sight to the Sun. The net effect is that the Sun is "reddened" slightly, because of the removal of blue light, while the sky away from the Sun appears blue. In outer space, where there is no atmosphere, there is no Rayleigh scattering of sunlight, and the sky is black (although, as we will see in Chapter 18, light from distant stars is reddened in precisely the same way as it passes through clouds of interstellar gas and dust.)
At dawn or dusk, with the Sun near the horizon, sunlight must pass through much more atmosphere before reaching our eyes—so much so, in fact, that the blue component of the Sun's light is almost entirely scattered out of the line of sight, and even the red component is diminished in intensity. Accordingly, the Sun itself appears orange—a combination of its normal yellow color and a reddishness caused by the subtraction of virtually all of the blue end of the spectrum—and dimmer than at noon. At the end of a particularly dusty day, when weather conditions or human activities during the daytime hours have raised excess particles into the air, short-wavelength Rayleigh scattering can be so heavy that the Sun appears brilliantly red. Reddening is often especially evident when we look at the westerly "sinking" summer Sun over the ocean, where seawater molecules have evaporated into the air, or during the weeks and months after an active volcano has released huge quantities of gas and dust particles into the air—as was the case in North America when the Philippine volcano Mount Pinatubo erupted in 1991. |
MORE PRECISELY 7-1 Why Is the Sky Blue?
