sky. The Earth, unlike the moon or space, has a protective blanket of gases, water and dust called an atmosphere. The blue color of the sky is a result of sunlight passing through and interacting with the atmosphere.
Question submitted by: J.W. Strutt of Okemos
The sky is not blue everywhere. Pictures taken in outer space show the Space Shuttle against a black background. Lunar scenes portray astronauts against a black sky. The Earth, unlike the moon or space, has a protective blanket of gases, water and dust called an atmosphere. The blue color of the sky is a result of sunlight passing through and interacting with the atmosphere.
If you look at sunlight passing through a prism, you will find that white sunlight is actually composed of a rainbow of colors. As the sunlight passes through the atmosphere, the atmosphere selectively filters blue light using a process called `scattering'. Scattering of light can be simply thought of as a light wave bouncing off of the air, water and dust particles which make up the atmosphere. An example of the scattering of sunlight can be seen in a smoke filled room. When sunlight enters the room, smoke particles scatter the light, resulting in shafts of visible light. The smoke particles, however, have little effect on the color of the scattered light.
As we have mentioned in earlier columns, light comes in different `sizes' which we call wavelengths. Different colors of light have different wavelengths, with red light having a long wavelength and blue light having a short wavelength. Selective scattering (preferential scattering of one color light over the other colors) occurs when the particles are much smaller than the wavelengths of the colors. Only one color is then scattered, and the atmosphere appears to be that color. In the case of our atmosphere, shorter wavelengths (blues) are scattered much more strongly than longer wavelengths (reds). As the sunlight passes through the atmosphere, blue light preferentially scatters and becomes visible to the eye. Large scale changes in the composition of the atmosphere - such as dust particles resulting from volcanic eruptions and particulate matter from air pollution - can also change the color of the sky.
Although the atmosphere is essential for supporting life on this planet, scattering of sunlight from the particles in the atmosphere is also responsible for our unique blue sky.
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