Light from the Sun is made up of several different colours, each of which has its own wavelength. The wavelength of the blue part of the Sun’s light is shorter than the size of an oxygen atom. When the blue light waves hit the oxygen atoms in the Earth’s atmosphere, they are scattered, making the sky appear blue. The light waves of other colours (with greater wavelengths than blue) are also affected, but blue waves are scattered more than most.
To understand why the sky is blue, we need to consider the nature of sunlight and how it interacts with the gas molecules that make up our atmosphere. Sunlight, which appears white to the human eye, is a mixture of all the colors of the rainbow. For many purposes, sunlight can be thought of as an electromagnetic wave that causes the charged particles (electrons and protons) inside air molecules to oscillate up and down as the sunlight passes through the atmosphere. When this happens, the oscillating charges produce electromagnetic radiation at the same frequency as the incoming sunlight, but spread over all different directions. This redirecting of incoming sunlight by air molecules is called scattering.
The blue component of the spectrum of visible light has shorter wavelengths and higher frequencies than the red component. Thus, as sunlight of all colors passes through air, the blue part causes charged particles to oscillate faster than does the red part. The faster the oscillation, the more scattered light is produced, so blue is scattered more strongly than red. For particles such as air molecules that are much smaller than the wavelengths of visible light the difference is dramatic. The acceleration of the charged particles is proportional to the square of the frequency, and the intensity of scattered light is proportional to the square of this acceleration. Scattered light intensity is therefore proportional to the fourth power of frequency. The result is that blue light is scattered into other directions almost 10 times as efficiently as red light.
When we look at an arbitrary point in the sky, away from the sun, we see only the light that was redirected by the atmosphere into our line of sight. Because that occurs much more often for blue light than for red, the sky appears blue. Violet light is actually scattered even a bit more strongly than blue. More of the sunlight entering the atmosphere is blue than violet, however, and our eyes are somewhat more sensitive to blue light than to violet light, so the sky appears blue.
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