| Why is the Sky Blue? To understand the answer to this question, first understand what the sky looks like on the moon. The moon is 81 times lighter than the earth (yet it has 28% of the diameter so you will weigh 15% of what you do on earth on the moon). It is not massive enough and therefore does not have a strong enough gravitational field to hold an atmosphere. Because most air molecules travel really fast (most at supersonic speeds and lighter elements travel even faster)the moon’s small gravity is not enough to keep them from shooting off into space. So it has no atmosphere and thus the day time sky from the lunar surface looks just like our night time sky. You can see the stars for 24 hours a day on the moon. The earth’s gravitational field is strong enough to hold an atmosphere, so our skies are filled with all but the lightest gases (low mass atoms like those of hydrogen and helium are light enough to escape the earths gravitational field). During the day time we cannot see the stars because the gas molecules in our atmosphere disperse much of the light from the sun in random directions. This is ambient light. A very small percentage of the total light that travels through our atmosphere is actually absorbed by it. However, enough light is scattered by the atmosphere to visually drown out the light from daytime stars. During the night time we can see the stars because the sun is no longer present to illuminate the atoms in our atmosphere. The light that makes up the ambient light in our atmosphere comes from the sun, which emits white light. White light contains all of the wavelengths of light that are in the visible spectrum (the ones that we can see). The nitrogen and oxygen atoms in our atmosphere absorb many of these different wavelengths of sunlight. They absorb less and scatter more of the higher frequency wavelengths (bluish light) and therefore there is more blue light bouncing around from atom to atom in the sky. The difference in wavelength causes blue light to be scattered nearly ten times more than red light. Lord Rayleigh of England (b. 1842) observed this phenomena and so it is called Rayleigh scattering (and is also known as the Tyndall effect). The color that is scattered most is violet because it has the shortest wavelength of any visible light. The sky is blue to us only because we have better receptivity in our retinas for blue light than for violet light. |
Organization for the Advancement of Interdisciplinary Learning |
Physics
| Organization for the Advancement of Interdisciplinary Learning |