Post by Icarus on Jul 8, 2008 9:57:08 GMT -5
Ok. This post may seem like a relatively “elementary” thing, but in the past week, I have heard this argument twice, so I am going to post this article from an Astronomy book to clarify it.
Why Is The Sky Blue?
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 highly sensitive to the wavelength of the light involved.
Rayleigh found that blue light is much more easily scattered than red light, basically because the wavelength of blue light (400nm) is closer to the size of air molecules than the wavelength of red light (700nm). He went on to prove mathematically, on the basis of the laws of electromagnetism, that the amount of scattering is inversely proportional to the fourth power of the wavelength.
Rayleigh’s formula applies to scattering by particles (such as molecules) that are smaller than the wavelength of the light involved. Larger particles, such as dust, also preferentially scatter blue light, but by an amount that depends only inversely on the wavelength.
Example: Let’s compare the relative scattering of blue (400nm) and red (700nm) light by atmospheric molecules and dust. For Rayleigh scattering, blue light is scattered (700/400) (to the 4th power) about 9.4 times more efficiently than red light. That is, blue photons are almost 10 times more likely to be scattered out of a beam of sunlight (taken out of the forward beam and redirected to the side) than are red photons. For scattering by dust, the corresponding factor is (700/400)=1.75-not as big a differential, but still enough to have a large effect when the air happens to be particularly dirty.
When the Sun is high in the sky, the blue component of incoming sunlight is scattered 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 first figure. Red or yellow light is scattered relatively little and arrives at our eyes predominantly along the line of sight to the Sun. This 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 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.
Source: Astronomy Today, 5th edition, 2005
Why Is The Sky Blue?
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 highly sensitive to the wavelength of the light involved.
Rayleigh found that blue light is much more easily scattered than red light, basically because the wavelength of blue light (400nm) is closer to the size of air molecules than the wavelength of red light (700nm). He went on to prove mathematically, on the basis of the laws of electromagnetism, that the amount of scattering is inversely proportional to the fourth power of the wavelength.
Rayleigh’s formula applies to scattering by particles (such as molecules) that are smaller than the wavelength of the light involved. Larger particles, such as dust, also preferentially scatter blue light, but by an amount that depends only inversely on the wavelength.
Example: Let’s compare the relative scattering of blue (400nm) and red (700nm) light by atmospheric molecules and dust. For Rayleigh scattering, blue light is scattered (700/400) (to the 4th power) about 9.4 times more efficiently than red light. That is, blue photons are almost 10 times more likely to be scattered out of a beam of sunlight (taken out of the forward beam and redirected to the side) than are red photons. For scattering by dust, the corresponding factor is (700/400)=1.75-not as big a differential, but still enough to have a large effect when the air happens to be particularly dirty.
When the Sun is high in the sky, the blue component of incoming sunlight is scattered 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 first figure. Red or yellow light is scattered relatively little and arrives at our eyes predominantly along the line of sight to the Sun. This 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 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.
Source: Astronomy Today, 5th edition, 2005