Campfire Smoke: Blue Trees, Yellow Sky

[Pranav Jha]

Why does smoke from a campfire look blue against trees near the ground but yellow against the sky?

 

[schip666!]

I think you are probably seeing reflected light when looking against the trees, and transmitted light when looking against the sky. If smoke reflects blue it will pass more yellow...

 

[Pranav Jha]

transmitted light when looking against the sky. If smoke reflects blue it will pass more yellow...

Why does it reflect blue but transmit yellow on the first place? Secondly, why would it seem yellow if it transmitted more yellow? If it is reflecting blue, shouldn't it appear bluish?

 

[schip666!]

Why does it reflect blue but transmit yellow on the first place? Secondly, why would it seem yellow if it transmitted more yellow? If it is reflecting blue, shouldn't it appear bluish?

Ok, let's think this through, backwards...

Start with a piece of glass that has a coating that reflects blue light.

When you have it lying on a table what you see is the light that is reflected, so it looks blue.

Then you pick it up and hold it between the light and your eye. Since (some of) the blue is being reflected what you see coming through the glass is what's left over. In the simplest terms what's left is red and green which we see as yellow.

I know the red+green = yellow thing is a bit hard to take, but that's the way Color Theory works due to how our eyes function. Probably wiki will help in this regard.


Now, why would smoke reflect blue? I dunno. Why does blue paint reflect blue? Generally the explanation involves having the paint absorb red and green and only re-emit blue. So the paint molecules have an affinity for red/green light, which means that their electrons can catch photons with those energies and eventually change them into heat.

That isn't such a good mechanism for my reflection only explanation, but I'm pretty sure something similar is going on. Another possibility is: when looking through smoke against the sky your eye compares it to the blue background and sees it as more yellow.

Maybe someone who knows more about gasses and suspensions can chime in here? I really should have stopped at color theory...

 

[diazona]

It's probably because the transitions between electron energy levels in the molecules that make up smoke have frequencies that correspond to blue light, more than red or green. That's the usual reason why things have the colors they do.

 

[Cleonis]

Why does smoke from a campfire look blue against trees near the ground but yellow against the sky?

Expanding on what schip666 has written:

Some of the smoke particles are small enough to scatter light. The smoke particles are very diluted in the air, but a small portion of the light traveling through the smoke is scattered. The blue portion of the spectrum is scattered more strongly than the red portion.

The same scattering takes place in the atmosphere as a whole. As the sun is setting (or when it's rising), the light has a long way to travel through the atmosphere before reaching our eyes. The red part of the spectrum is less likely to be scattered, so at sunset more of the red reaches us. That is why the setting Sun looks red.

During the day, when we look at a part of the sky where the Sun is not, we see light that has been scattered. Looking at the sky you see light that hasn't traveled from the Sun in a straight line; it has "bounced off" at least once. The blue part of the spectrum is more likely to be scattered, so the sky is has a blue tint.

Looking at the sky with smoke laden air in between you get the same effect that renders the setting Sun red. (Color recognition by the retina proceeds by comparison. Your eyes see less blue in the light that traveled through the smoke laden air, as compared to "clean" light from the surrounding sky. That suffices.)

Finally, there is the case of looking at smoke laden air against a dark background, with the smoke laden air illuminated by light from the sky. More blue light than red light is scattered all the way back.

 

[schip666!]

ah...scattering...much better explanation than reflection...but maybe there's a bit of both?
thx

 

[Cleonis]

ah...scattering...much better explanation than reflection...but maybe there's a bit of both?
thx

Well, we call it 'reflection' when there is some preservation of direction of motion. A beam of light striking a flat, reflective surface remains a beam.

Any white surface is absorbs very little of the light that strikes it, but instead of preserving directional information (like a mirror does) the light is scattered. Matted glass scatters light because instead of an orderly, smooth surface it presents a rough, chaotic surface. Smoke particles in air are totally chaotic, so I think only scattering is involved.


That leaves the matter of how that "scattering" is supposed to take place. We are familiar with the concept of light being diffracted when it passes through very small holes (holes close in size to the wavelength of light). When there are particles in the air with a size in the order of the wavelength of light there are diffraction effects too (but in a very "diluted" form of course.)

Interestingly, it's not necessary to think in terms of photons hitting smoke particles or so, classical wave mechanics predicts the same.

 

[Andy Resnick]

That leaves the matter of how that "scattering" is supposed to take place. We are familiar with the concept of light being diffracted when it passes through very small holes (holes close in size to the wavelength of light). When there are particles in the air with a size in the order of the wavelength of light there are diffraction effects too (but in a very "diluted" form of course.)

"Mie scattering" is the appropriate theory:

http://www.google.com/url?sa=t&sour...sg=AFQjCNEkDzNKKI4TZerWQajENqinP6Mrhw&cad=rja

 

[Pranav Jha]

Expanding on what schip666 has written:

Some of the smoke particles are small enough to scatter light. The smoke particles are very diluted in the air, but a small portion of the light traveling through the smoke is scattered. The blue portion of the spectrum is scattered more strongly than the red portion.

don't the smoke particles normally scatter red light (longer wavelength light) compared to the shorter wavelength light?