Why is the sky blue?

Mk

What are some reasons that the sky is blue?

Last edited:
Related General Physics News on Phys.org

Cosmo16

It is the way the light bends as it passes through the atomosphere. (Or somthinkg like that)

stalefish

http://en.wikipedia.org/wiki/Rayleigh_scattering

The amount of scattering depends on the frequency of the light. Blue light, having a higher frequency, scatters more than lower frequencies (Which pretty much pass straight through the atmosphere). This scattering of blue light more than the other visible frequencies results in us seeing the sky as blue.

Mk

So is air blue? Water is generally blue, the greater the quantity the bluer it seems to get, is air the same way?

stalefish

Water itself isn't blue, really. Hey, it's the liquid we drink all the time!

Last edited:

The_Thinker

The reason water is blue, is because of the reflection of the blue sky on the water... thats all... water isn't blue...

FredGarvin

Why is the sky blue? If it were green we wouldn't know where to stop mowing.

stalefish said:
http://en.wikipedia.org/wiki/Rayleigh_scattering
The amount of scattering depends on the frequency of the light. Blue light, having a higher frequency, scatters more than lower frequencies (Which pretty much pass straight through the atmosphere). This scattering of blue light more than the other visible frequencies results in us seeing the sky as blue.
This peeling off of the lower frequencies in the spectrum (blues) is called Rayleigh scattering.

The_Thinker said:
The reason water is blue, is because of the reflection of the blue sky on the water... thats all... water isn't blue...
No, actually Rayleigh scattering is at least partially responsible for water being blue too. The deeper you go in water the more of the spectrum is bounced out. At 33 feet you've lost the color red. At 66 feet you've lost orange.

This is the reason divers need flashlights that use the whole spectrum. Otherwise they would misidentify objects.

Chi Meson

Homework Helper
Water absorbs radiant energy ("light"). Water will more quickly absorb light with longer wavelengths. Water absorbs microwave and infra red wavelengths almost immediately, and visible red relatively quickly. If the red part of the spectrum is absorbed, then the gree/blue colors remain.

gree-to-blue light is going to be scattered in the upper layers of water, so when viewed from above, water appers to be green-blue.

The sky is blue due to scattering entirely. The blue light from the sun hits air molecules and is absorbed mometarily, then re-emitted in a random direction. REd light does not get so easily absorbed/re-emitted and thus maintains more of a straight line path, hence orange-red sunsets.

The notion of water reflecting the sky is an oft-quoted misconception that is unfortunately still taught in grade-school "science" textbooks.

Mk

So is water blue because water is blue? Or is it scattering? Or something else?

Tide

Homework Helper
An important factor in the sky being blue is the presence of fluctuations in density and temperature (i.e. nonuniformity). Without such fluctuations the the Rayleigh fields will be cancelled at every point.

G

GENIERE

FredGarvin said:
Why is the sky blue? If it were green we wouldn't know where to stop mowing.

You made me spill my coffee.

Mk

GENIERE said:
You made me spill my coffee.
Did it scatter? Or was it something else?

dextercioby

Homework Helper
The best explanation (also with numerical values to get an idea) one can find in any of the 3 editions of J.D.Jackson's text on CED.

Daniel.

Chi Meson

Homework Helper
Mk said:
So is water blue because water is blue? Or is it scattering? Or something else?
Things are the color that they are due to teh frequencies of light that emerge from them. Ignoring whatever might be in the water (algae, mud, red tide etc) oceans are gree-blue because that is the light that scatters back out of the oceans.

"Why is the sky blue?"

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving