Why is the sky blue? It's so simple.

[groovychk]

Rayleigh scattering
From Wikipedia, the free encyclopedia.

Rayleigh scattering causing a reddened sky at sunset

Rayleigh scattering (named after Lord Rayleigh) is the scattering of light by particles much smaller than the wavelength of the light. It occurs when light travels in transparent solids and liquids, but is most prominently seen in gases. Rayleigh scattering of sunlight from particles in the atmosphere is an early approximation of the reason for why the light from the sky is blue.

The amount of Rayleigh scattering that occurs to a beam of light is dependent upon the size of the particles and the wavelength of the light; in particular, the scattering coefficient, and hence the intensity of the scattered light, varies inversely with the fourth power of the wavelength, a relation known as the Rayleigh law. Scattering from particles larger than about a tenth of the illuminating wavelength is handled by Mie theory.

The intensity I of light scattered by a single small particle from a beam of light of wavelength λ and intensity I0 is given by:



where R is the distance to the particle, θ is the scattering angle, n is the refractive index of the particle, and d is the diameter of the particle.

The angular distribution of Rayleigh scattering, governed by the (1+cos2 θ) term, is symmetric in the plane normal to the incident direction of the light, and so the forward scatter equals the backwards scatter. Integrating over the sphere surrounding the particle gives the Rayleigh scattering cross section σs:



The Rayleigh scattering coefficient for a group of scattering particles is the number of particles per unit volume N times the cross-section.


The strong wavelength dependence of the scattering (~λ-4) means that blue light is scattered much more than red light. In the atmosphere, this results in blue photons being scattered across the sky to a greater extent than photons of a longer wavelength, and so one sees blue light coming from all regions of the sky whereas the rest is still mainly coming directly from the Sun. It should be noted that, despite the use of the term photon, Rayleigh scattering was developed prior to the invention of quantum mechanics and is not based fundamentally in modern theory about the interaction of light with matter. Nevertheless, Rayleigh scattering is a fair approximation to the manner in which light scattering occurs within various media.

A notable exception occurs during sunrise and sunset, when the Sun's light must pass through a much greater thickness of the atmosphere to reach an observer on the ground. This extra distance causes multiple scatterings of blue light, but relatively little scattering of red light; this is seen as a pronounced red-hued sky in the direction towards the sun.

If the size of particles are larger than the wavelength of light, light is not separated and all wavelengths are scattered as by a cloud which appears white, as do salt and sugar. For scattering by particles similar to or larger than a wavelength, see the article on Mie scattering.

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HAHAHAHA - I think "Just because, kid." will do for now.

Comments

[hwrnmnbsol.livejournal.com]

I figured you were bored, but I didn't know *how* bored.

[groovychk.livejournal.com]

My mind goes weird places. And YES I'm bored bored bored. 75% of the time at least. I need to be running the universe or something.

How I got to the Blue Sky deal.... I was responding to a post that an LJ friend left about her and her son drawing out the solar system on the patio with chalk and the question arose of whether the newly discovered object in the outer solar system (which is bigger than Pluto and about three times further from the Sun) had a moon or several or what. They settled on three. I was being a space geek and pointing out that they have found a moon and also added in information on Quaoar and Sedna as well as a link to the Extrasolar Vision webpage which lists all 160 planets discovered outside the solar system and has cool visualizations. I was then looking at that site and checking out some of the water vapor jovian planets and reading the description which said they'd appear blue like Neptune from the Rayleigh Scattering.... Wheee. Busy brain.

[hwrnmnbsol.livejournal.com]

My color-blindness keeps me from being overly interested in planets' colors, but the moon question is very interesting to me. Because, you know, moons are cool.

I'm supposed to be busy but I can't get focussed, so I'm listening to the Astros game and fiddling. I need to be productive! grrr...

[drkbish.livejournal.com]

I prefer the Night Court answer.

"Because if it was green, you wouldn't know where to stop mowing your lawn."

[groovychk.livejournal.com]

awesome :-)

[groovychk.livejournal.com]

seriously though - it is because the sky is scattering blue light from the sun.

http://www.badastronomy.com/bad/misc/blue_sky.html

easier read

[dagulfsghost.livejournal.com]

scary thing is, i got it.

[groovychk.livejournal.com]

you get tonight's bozo button! :-> the real trick is to try to explain it... not only to another adult - but to a kid! :-)

arghhh Physics!

[(Anonymous)]

I used to write that equation up on the blackboard in the observatory
that I worked at. It looked cool and would impress people when I could explain what it all meant. I used to get the "why is the sky blue question several times a day, as all the school tours would come by. I also used to put a few pics of mars up to show the difference the atmosphere makes. Ahhh I miss that job.

-Zaphod the phsyics major.

Re: arghhh Physics!

[groovychk.livejournal.com]

Hehe - Sounds like a very neat job.

[razormaid.livejournal.com]

*bows down*
I can't wait til Max asks that question...

[groovychk.livejournal.com]

That will be very neat I'd imagine :-)