The clear sky appears blue because of a phenomenon called Rayleigh scattering, where sunlight is scattered by tiny air molecules in the Earth's atmosphere, and blue light is scattered more effectively than other colors.
Understanding Rayleigh Scattering
Rayleigh scattering is the scattering of electromagnetic radiation (including visible light) by particles of a wavelength much smaller than the wavelength of the radiation. In the Earth's atmosphere, these particles are primarily nitrogen and oxygen molecules.
Why Blue Light is Scattered More
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Wavelength and Scattering: The amount of scattering is inversely proportional to the fourth power of the wavelength of light. This means shorter wavelengths (like blue light) are scattered much more strongly than longer wavelengths (like red light). Since the wavelength of red light is about 1.8 times greater than blue light, blue light is scattered much more intensely.
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Effect on Sky Color: When sunlight enters the atmosphere, the shorter blue wavelengths are scattered in all directions by these particles. This scattered blue light reaches our eyes from all directions, making the sky appear blue.
Why Not Violet?
While violet light has an even shorter wavelength than blue light, sunlight contains less violet light, and our eyes are also less sensitive to violet. Thus, the sky appears primarily blue, not violet.
The Sunset Effect
At sunrise and sunset, when sunlight passes through a greater distance of the atmosphere, most of the blue light has already been scattered away. The longer wavelengths, such as red and orange, are less scattered and can reach our eyes, giving us the beautiful red and orange colors of the sunset and sunrise.
In summary, the blue color of the sky is a direct result of Rayleigh scattering, where blue light is scattered more effectively than other colors due to its shorter wavelength.
