Blue Smoke, Red Sun

I was living in Indiana back in June when I got a surprise lesson in optics, simply by looking out my window to see the golden-orange glow of sunset bathing our lawn. That’s odd, I thought to myself, seems like the day just flew by.

I checked my watch; it was 3 P.M. I blinked hard, wondering to myself what could be going on. I checked my phone; still 3 P.M. I stepped outside for a better look around.
There was no mistaking it; this was the kind of color scheme I was used to seeing an hour or two before the last light of the day, but the sun was hanging stubbornly far above the horizon, at its usual 3 o’clock position.

All at once, two memories hit me in tandem, and I grinned as I realized I knew exactly what was happening. The first memory was from the prior week; I had been driving, and the car in front of me belched out a cloud of blue oil-smoke from its tailpipe as it accelerated away from a stoplight. Driving along, I wondered to myself why it should be blue and not white or grey; in the past, I had always chalked up smoke’s occasional bluish tinge to a trick of the light, or my perception, but here I could see that it was a very definite blue. When I got home I began to do some research, and discovered that what I had seen was known as the Tyndall effect, a preferential scattering of blue light that occurs when the size of particles suspended in a medium is close to the wavelengths of the visible spectrum. Grey and white clouds, I discovered, occur when the particles are much larger than the wavelength of light they’re scattering. 
The second memory was of a news headline that I’d read that morning: wildfires raging across southern Canada.  To this day I can’t be sure if it was real or imagined, but in that moment I was certain I could smell the faintest trace of wood smoke in the air, reminiscent of campfire nights. I realized that there was a haze blanketing the region, and from the outside it must look a little blue. I was struck with a deep sense of wonder at the symmetry inherent in this inversion of perspective; that I was standing in the red shadow of a blue cloud.

Wildfire smoke blows south from Alberta, creating a thick haze that reaches as far as the eastern seaboard.
Photo courtesy NASA
I couldn’t have asked for a better reminder of why I love physics. All at once I was three inches tall, squinting up at the sun through a cloud of oil-smoke from a tailpipe. I was thirty-thousand feet in the air, looking down at the Midwest. I wondered if people with blue eyes see color a little differently than people with brown ones; the iris’ blue color comes from the very same Tyndall effect that I was witnessing here on a massive scale. I called out to my mom to come look, and we stood there admiring the bizarre beauty of the light as I explained what we were seeing.
So check out our Physics in Pictures article on the Tyndall Egg, and next summer, if the fifth of July brings a clear night, look for the moon. If it rises looking rusty like it did this summer, you’ll know why.

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