Skip to main content

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.


Popular Posts

How 4,000 Physicists Gave a Vegas Casino its Worst Week Ever

What happens when several thousand distinguished physicists, researchers, and students descend on the nation’s gambling capital for a conference? The answer is "a bad week for the casino"—but you'd never guess why.

Ask a Physicist: Phone Flash Sharpie Shock!

Lexie and Xavier, from Orlando, FL want to know: "What's going on in this video ? Our science teacher claims that the pain comes from a small electrical shock, but we believe that this is due to the absorption of light. Please help us resolve this dispute!"

The Science of Ice Cream: Part One

Even though it's been a warm couple of months already, it's officially summer. A delicious, science-filled way to beat the heat? Making homemade ice cream. (We've since updated this article to include the science behind vegan ice cream. To learn more about ice cream science, check out The Science of Ice Cream, Redux ) Image Credit: St0rmz via Flickr Over at Physics@Home there's an easy recipe for homemade ice cream. But what kind of milk should you use to make ice cream? And do you really need to chill the ice cream base before making it? Why do ice cream recipes always call for salt on ice?