User:Milton Beychok/Sandbox

Why the sky is blue

Light is electromagnetic radiation that travels in waves of vibrating electric and magnetic fields and is a small part of a larger range of vibrating electromagnetic fields called the electromagnetic spectrum. Visible light is electromagnetic radiation visible to the human eye and is only a small part of the electromagnetic spectrum.

Light from the sun looks white, but is actually a combination of many colors. The colors blend continuously into one another. At one end of the spectrum are the red colors which have the highest wavelength and therefore the lowest frequency. At the other end of the spectrum are the blues and violets with the lowest wavelengths and therefore the highest frequency.

As light from the sun travels through Earth's atmosphere, it bumps into a bit of dust or a water droplet or a gas molecule. Dust particles and water droplets are very much larger than the wavelength of visible light. When light bumps into them, it gets reflected and bounced off in a different direction but the reflected light still appears white because it still contains all of the same colors it had before it was reflected.

However, gas molecules are much smaller than the wavelength of visible light. When light bumps into a gas molecule, it behaves differently than it does when it bumps into a dust particle or a water droplet in that some of the colors in the light are actually absorbed by the molecule. Subsequently, the molecule radiates or releases the light in a different direction. The colors in the radiated light are the same colors that were absorbed.

But the higher frequencies (blues) are absorbed more often than the lower frequencies (reds). This process is called Rayleigh scattering. (It is named after Lord John Rayleigh, an English physicist, who first described it in the 1870's.)