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Why is the sky blue; why is the sea blue or green?


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#1 jimmydasaint

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Posted 7 July 2010 - 07:13 PM

Simple question, but why is the sky blue but we still get loads of white light coming through the atmosphere and we don't all appear blue.

Moreover, if the sky is a brilliant blue and the sea reflects the colour, why is the sea sometimes green?

All simple answers welcomed.
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#2 insane_alien

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Posted 7 July 2010 - 07:47 PM

raleigh scattering.

blue light gets scattered more than red. we still get a lot of white because most of the light is unaffected, otherwise it'd like be staring into the sun wherever you looked.

the sea is blue because water is blue, the more water around, the bluer it is, the green colour usually comes from stuff living in it.
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#3 Genecks

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Posted 9 July 2010 - 05:55 AM

Because my eyes have evolved to perceive it as blue.
And that's a fact, jack.
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#4 jimmydasaint

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Posted 9 July 2010 - 09:21 PM

OK. Thanks for the answers. I am reading up on Rayleigh scattering and found this easy-to-read site. Thank God! I can now explain it to myself thus:

Gas molecules are smaller than the wavelength of visible light. If light bumps into them, it acts differently. When light hits a gas molecule, some of it may get absorbed. After awhile, the molecule radiates (releases, or gives off) the light in a different direction. The color that is radiated is the same color that was absorbed. The different colors of light are affected differently. All of the colors can be 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.)

WHY IS THE SKY BLUE?

The blue color of the sky is due to Rayleigh scattering. As light moves through the atmosphere, most of the longer wavelengths pass straight through. Little of the red, orange and yellow light is affected by the air.

However, much of the shorter wavelength light is absorbed by the gas molecules. The absorbed blue light is then radiated in different directions. It gets scattered all around the sky. Whichever direction you look, some of this scattered blue light reaches you. Since you see the blue light from everywhere overhead, the sky looks blue.


Link

Genecks thanks for the clues dude (half rhyme :) ).

Edited by jimmydasaint, 17 July 2010 - 03:39 PM.

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#5 StringJunky

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Posted 9 July 2010 - 10:29 PM

raleigh scattering.

blue light gets scattered more than red. we still get a lot of white because most of the light is unaffected, otherwise it'd like be staring into the sun wherever you looked.

the sea is blue because water is blue, the more water around, the bluer it is, the green colour usually comes from stuff living in it.


I think you've cocked up there mate. ;). My guess is all the colours are absorbed except blue which is scattered.

If our eyes had a sensitivity in the violet portion of the spectrum equal to the blue portion the sky would appear more violet than we actually see it because indigo and violet is scattered just as much if not more than blue. Our brain interprets the blue portion only, I suppose because there's not enough violet sensitivity to have an effect.

Edited by StringJunky, 9 July 2010 - 10:49 PM.

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#6 CharonY

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Posted 9 July 2010 - 10:47 PM

Actually IA is correct. Water has a (very) low absorption of the reddish parts within the visible light spectrum.
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#7 StringJunky

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Posted 9 July 2010 - 11:01 PM

Actually IA is correct. Water has a (very) low absorption of the reddish parts within the visible light spectrum.


Is it enough to give it it's blue colour with increasing volume and not down to scattering?
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#8 jimmydasaint

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Posted 20 July 2010 - 06:53 PM

Thank you for all the replies. Having done some reading, this is my take on it:

1. Light from the sun is white because it contains different frequencies of light mixed together.
2. The colours are: red, orange yellow, green, blue, indigo and violet.
3. Red light has the longest wavelength and lowest frequency (number of waves passing by a fixed place per second, also called hertz, Hz). Blue light has the shortest wavelength out of the light spectrum and the highest frequency.
4. When the mixture of waves hit the Earth's atmosphere, they are scattered or reflected by dust particles, which are relatively large.
5. When the light hits gas particles in the air (a mixture of nitrogen and oxygen for the most part), it behaves differently.
6.Red light, orange light, yellow light and green light get through the gases being absorbed and radiated without much problem.
7. Blue light is absorbed by the gas particles but then scattered afterwards in the same manner as a card dealer in a casino scatters the cards amongst the players.
8. We see this scattering all around us.
9. The sun appears yellow due to the mixture of the colours of waves that get through the gases in the atmosphere.
10. The sea appears blue because it reflects light from the sky, AND, because some of the light waves of the spectrum are absorbed and some are scattered in the same manner as the sky. Where the sea is green, it is due to different particles in it.

I hope this makes sense, because this is the way I explained this simple phenomenon to myself.
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#9 Sisyphus

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Posted 20 July 2010 - 07:35 PM

I'd say that that is mostly right, except:

2) We see those distinct colors because of how our eyes work, not because they are in fact distinct. Sunlight is a continuous spectrum.
3)It's violet that has the shortest wavelength, not blue.
10) I think you can be more specific than just particles. The ocean is full of green algae.
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I think you'll find it's a bit more complicated than that.

#10 jimmydasaint

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Posted 20 July 2010 - 07:42 PM

I stand corrected Sisyphus. Yes, of course violet light has the shortest wavelength and that the sea is full of creatures :rolleyes: However, does the type of dissolved salt not also have an effect on sea colour?
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#11 lemur

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Posted 8 September 2010 - 12:55 AM

I stand corrected Sisyphus. Yes, of course violet light has the shortest wavelength and that the sea is full of creatures :rolleyes: However, does the type of dissolved salt not also have an effect on sea colour?

You can test your hypothesis by dissolving salt in water and observing whether its color or other reflection/refraction properties change. If you time your experiment well, you could follow it up with a pasta dinner or a nice soup. Have fun!
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