Tuesday, December 11, 2012

What color is smoke?

What color is smoke? The simple answer is that it depends on the color of the background you see it against.

Smoke against a dark background appears blue.

Against a light sky the same smoke will appear orange. Sometimes an observer will see smoke passing upward from a dark forest background to a light sky background, and it will actually appear to to shift from one color to another.

These colors are not an intrinsic quality of the smoke in the same way green is intrinsic to green colored glass. The color is a result of the behavior of the light rays as they pass through the smoke. 

As M. Minnaert explains: "Against a dark background, the smoke is illuminated by rays from the sun falling on it obliquely from all directions except from behind; these rays are scattered by the smoke in every direction and some of the scattered rays enter our eyes and make the smoke visible. The particles which make up the smoke scatter blue light much more than red or yellow: therefore we see the smoke as blue. On the other hand, when the background is bright we see the smoke by transmitted light and it appears yellow because the blue in the incident white light has been scattered in all directions, very little can reach our eyes, and only the yellow and red remain to be transmitted and give color to smoke."

When the particle size of smoke increases (such as when each particle is encased in a tiny droplet of water) it loses these scattering properties, and the smoke appears relatively white. Smoke can also take on specific colors if certain chemicals are introduced into it (think colored smoke bombs).

(Direct link to video--Start at 28 minutes). In this video, the light-scattering properties of smoke are shown in a vivid classroom demonstration by Professor Emeritus Walter Lewin at MIT, author of For the Love of Physics.
Recommended books: The Nature of Light and Colour in the Open Air (Dover Books on Earth Sciences) by M. Minnaert

For the Love of Physics: From the End of the Rainbow to the Edge of Time - A Journey Through the Wonders of Physics by Walter Lewin

Forest fire photo from Crater Lake Institute
Thanks, Rob Nonstop


Ken Chandler said...

Fascinating, prior to your post I'd given it very little thought. Thank you Mr. Gurney. You've altered my perception forever.

Robnonstop said...

Thank you for posting the video, I actually sent an email about a specific light question to Dr.Lewin and he immediately explained it to me.

Aaron said...

Ok, I watched the whole video now I have a headache.lol (j/k, but it really is interesting how much artists can use physics to recreate a believable environment.)

Anonymous said...

Once again, it seems to boil down to painting what you actually see, not what you think you should see.

Anonymous said...

I've seen the sickly orange cast smoke billowing from brush fires nearby. I have wondered why the little tuft of smoke from the cabin in the 'Miners in the Sierras' painting I have on my desktop by Nahl and Wenderoth has a decidedly blue tinge. I noticed it in a Kinkade painting as well and wondered why the blue smoke. Now, not only do I understand the orange and blue smoke, I also understand why the sky is blue, clouds are white and sunsets are red!

What a cool guy and great showman, Walter Lewin is. Loved the 'rocket' and the cigarette demo. Passing this on to my son who is currently a physics supplemental instructor at his university. I may have to get him Lewin's book for Christmas. - mp

Unknown said...

This post came at a perfect time as I was just lecturing on color and light with my freshman class yesterday. Thanks so much for this James!

Diana Moses Botkin said...

What a delightful lecture by a very fascinating and knowledgeable scientist. It is a privilege and inspiration to enjoy Professor Lewin's last lecture, at age 75.

His comment during the Q&A about color perception not only being about physics but neuroscience, gives food for thought.

This post comes at a good time for me, also. I just read an old article about Maxfield Parish's technique using mostly transparent layers of pure color, so am thinking right now about such matters.

Anonymous said...

Diana, I was looking at my Maxfield Parrish book just yesterday! Exquisite color! - mp

Tyler J said...

To complicate things slightly, it also depends on what is burning.

Natural materials, such as wood, generally throw off the whiter-type smoke that more readily takes light.

Many man-made materials, such as polethylene foam in seat cushion, throws off a much darker (blacker) smoke.

If creating a piece from imagination, it would really be time well spent to find some good reference images for whatever is burning in your scene. Highly processed fuel (like oil, couches, paint) will generally have much darker smoke.

Michael said...

I had an instructor recommend using a touch of Alizarin Crimson in my white and gray mixture to paint believable shadows on the dark side of clouds. With my sporadic time painting clouds I've found it usually works great. Is there often this warm tone in cloud shadows? If so, is it for the same reason we see orange in smoke lit by transmitted light conditions?

James Pailly said...

I remember first reading about Rayleigh scattering in one of your books. As I was reading this post, I started thinking how this sounds like Rayleigh scattering, and then in the video Dr. Lewin uses that term almost immediately.

Thanks not only for teaching me something but also giving me an opportunity to prove (at least to myself) that I really understood it!

Salty Pumpkin Studio said...

Wonderful lecture, it doesn't matter the smoke part is a half hour +/_ into the video. I kept watching before, during and after.
Thank you for posting it

Jessica Casner said...

Great post as always, Mr. Gurney.

Bob Eggleton shared a post today on Facebook about a particular artist, John Brosio, who paints tornadoes. I wonder how big of a difference there is between smoke and dust, especially in the context of the time of day.

Here's a link about John's work: http://www.huffingtonpost.com/john-seed/john-brosio-tornadoes_b_2308777.html#slide=1896383

James Gurney said...

Thanks, Jessica,
I've seen John's work in an art magazine, and I love it, and I enjoyed his interview in H.P.

As far as smoke and dust, they have some things in common (creating more atmospheric lightening of distances), but dust usually has larger particles, so it doesn't scatter the light as much.