As a kid I would sit in the backseat and look up out the window and wonder why the sky was blue. But as an incredibly lazy kid, I never bothered to find out the answer until college. And it wasn't even me looking the answer up. We were given the explanation in p-chem lecture.
The skinny
The fat fat fattie
To understand (and appreciate!!) the blue sky phenomenon, we gotta cover a few main ideas....
The atmosphere
Think of the atmosphere as containing many molecules of different gases. Nitrogen and oxygen gas are the big ones, making up about 99% of the atmospheric gases. But carbon dioxide and argon gas are also present. There are trace amounts of other gases as well. (There's also water vapor and tiny particles of dust, etc, etc. present in the Earth's atmosphere too.)
The visible spectrum
Visible light travels in waves. The energy of radiation is dependent on its wavelength and frequency. The longer the wavelength, the shorter the frequency and smaller the energy.
The visible spectrum is the part of the electromagnetic spectrum that we can see. The wavelength range of the visible light corresponds to ~400-700 nanometers. Blue light has a wavelength of about 470 nanometers. And on the other end of the visible spectrum, red light has a wavelength of about 650 nanometers.
Light scattering
Photons can be absorbed or emitted by atoms. And photons can also be scattered. Rayleigh scattering is the elastic scattering of light responsible for the blue skies. Sunlight strikes the air molecules causing the light to be scattered. No energy is lost (elastic... same state) and no frequency changes occur. Lord Rayleigh calculated that the scattered intensity from super small molecules/particles (called dipole scatterers and gotta be much smaller than the wavelengths) to be:
where N is the number of scatterers, α is the polarizability, R is the distance to the particle, and θ is the scattering angle. From the equation you can see that the strength of the scattering is strongly dependent of the wavelength --> blue skies!!!
Now what happens if the scattering from molecules/particles are larger than a wavelength? That is when Mie scattering occurs. Unlike Rayleigh scattering, Mie scattering is not strongly wavelength dependent. Mie scattering is responsible for the white light from fog and the white glare you see around the sun.IMAGE SOURCE: Abel Leeman's Flower in the Sky from StockImages.com
SOURCE: Peter Atkin's Quanta, Matter, and Change
1 comments
Thank you for your through explanation on why the sky is blue!
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