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Small particles in the atmosphere scattering light cause the blue sky.
Blue SkyWhy is the sky blue? On a crystal clear day the sky is a lovely shade of Carolina blue. Many people wonder what causes this effect, but first let's consider a common myth about what does not cause the blue sky. Light Reflected off the Ocean Does Not Cause the Blue SkyThere is a common myth that light reflects off the oceans to cause the blue sky. To understand why this can not be correct, think about the implications. If the blue sky were in some way caused by oceans, then the sky would be a much richer shade of blue in coastal areas than in the middle of continents. For example on the North American continent California or any of the Atlantic coastal states would have bluer skies than Nebraska or Kansas, which are at the geographical center of North America. This author has personally verified that the sky can be just as blue in Nebraska and Kansas as in a coastal area. The oceans do not in any way cause blue skies. Particles in Earth's upper atmosphere scattering light cause the blue sky. Rayleigh ScatteringWhen light bounces, or reflects, off small particles, it is called scattering. Rayleigh scattering is the type of scattering that occurs when the particles are smaller than the wavelength of the light being scattered. In 1871, Lord Rayleigh worked out the mathematical law that describes how Rayleigh scattering works. Rayleigh found that shorter wavelength light is scattered much more efficiently than light at longer wavelengths. Mathematically, the amount of scattering is related to the fourth power of the wavelength. So blue light with a wavelength of about 400 nanometers is scattered about 10 times more efficiently than red light with a wavelength of about 700 nanometers. Earth's atmosphere contains lots of oxygen and nitrogen molecules which are smaller than the wavelength of light. It also contains lots of dust particles. They are however usually larger than the wavelength of light so the Rayleigh scattering law does not apply to the dust particles. It is the molecules that cause the blue sky. Why Is the Sky Blue?White light coming from the Sun contains all colors of light from red to blue. The molecules in Earth's atmosphere do not scatter much of the red light, but they do scatter a significant amount of blue light. The blue light might be scattered once or multiple times by different particles. A photon of blue light might bounce off one particle and be scattered in a random direction. It might then be scattered a second or even third time before coming down to Earth's surface. When we look up to the sky, we see blue light that appears to come from all parts of the sky. This light is the blue light from the sun that has been scattered from oxygen and nitrogen molecules in the atmosphere. The process is random, so blue light appears to originate from the entire sky. Knowing why the sky is blue can increase your enjoyment of a nice day. Remember the sky is Carolina blue not because God is a tarheel but because of Rayleigh scattering. Further ReadingWhy Are Sunsets and Sunrises Red Knight, R., Physics for Scientists and Engineers, Pearson, 2004.
The copyright of the article Why Is the Sky Blue? in Atomic/Molecular/Optical Physics is owned by Paul A. Heckert. Permission to republish Why Is the Sky Blue? in print or online must be granted by the author in writing.
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