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SYFY WIRE Bad Astronomy

Iridescence, like a candle in the wind

By Phil Plait
A slightly different airflow over the wick produces different patterns in the smoke and iridescent colors. Credit: Grover Schrayer

Science is beautiful.

I know, I say this a lot, but for one thing, it's true. And for another, I'm not convinced people know just how true it is. We go about our lives, perhaps engaging in a set of routines that we don't consider, don't really think about anymore.

Some of them are truly mundane, yet contain multitudes. For example, lighting a candle. Or more to the point, extinguishing one.

How does that contain both science and beauty? This way:

Colors, called iridescence, appear like magic in the smoke from a blow-out candle. But it’s not magic… it’s science. Credit: Grover Schrayer

Photographer Grover Schrayer captured this amazing moment with nothing more than a very nice camera, a macro lens, a flash, and a lot of patience. But he also had help from science … despite a lot of interference.

The physics here is actually extraordinarily complex. A lot of websites are saying this is a "rainbow effect," and I understand the desire to call it that, but that's not quite what's going on here. Rainbows are created when light from the Sun hits raindrops in the air. The light goes into the drop, refracts (bends) a little bit, reflects off the back of the drop, and comes back out (bending a bit more). The bending is what separates the colors in the rainbow, since different colors bend by slightly different amounts as they enter and leave the drop.

Also, this combination of reflection and refraction means that to see a rainbow, you have to have the Sun behind you, and the drops of water in front of you. The center of the rainbow is 180° around from the Sun.

In the case of this picture, the source of light was the flash on the camera, so the geometry is very close to being that of a rainbow. But look at the colors in the smoke; I see more than just the usual ROYGBV (red, orange, yellow, green, blue, violet) in a rainbow. I see pink, and magenta, and teal … this hints that there's more going on than simple refraction. Also, rainbows are created when the drops of water are decently big, a few millimeters wide. The smaller the droplet, the less intense the colors get. If they're too small, the bow becomes white (like in a fogbow, which are totally cool).

That's why what I think we're seeing here is actually iridescence. This is when colors are created by a more subtle process involving scattering of light. When light hits a very tiny droplet, it bounces off, like spraying a hose onto a ball. How that light bounces depends a lot on the wavelength (the color). What you see depends on how the light interferes with itself.

A slightly different airflow over the wick produces different patterns in the smoke and iridescent colors. Credit: Grover Schrayer

A slight diversion: Ever sat in a tub full of water and pushed yourself back and forth? You make a wave, and if you time your own motion with the timing of the wave, you can pump it way up, adding to it, making it splash really high. If you time it so that you're exactly out of phase of the wave, you can diminish it. This is because waves can add together crest to crest and get higher, what we call constructive interference. If they add trough to crest, the wave is diminished, and we call it destructive interference.

That's what I think is happening in the photo. The light is hitting tiny droplets, scattering off, and interfering with itself. The colors emerge due to this interference — as the waves interfere with each other the wavelengths and the waveform (the shape of the wave) change, creating the other colors. This is also angle-dependent; the angle between the light source (the flash in this case), the droplet, and your eye determines what color you see. That separates out the colors, creating the rainbow-like effect. This happens with clouds, too, something I post about with some (wait for it ...) frequency.

An iridescent cloud over Boulder, Colorado. The droplets near the edges of clouds like this are very tiny and all about the same size; with the Sun nearly behind them that sets up perfect conditions for iridescence. Credit: Phil Plait

But there's more science here! For example: What are the droplets then that are doing the lightcraft? Are they in the smoke?

Actually, they are the smoke! Part of it. Wax is a hydrocarbon, molecules that are a combination of (can you guess?) hydrogen and carbon. When you burn wax you add oxygen to the mix. If it burns completely, you wind up with only carbon dioxide (CO2) and water vapor (H2O). That's what usually happens while a candle is lit.

But when you blow the candle out the heat drops significantly. It doesn't burn the wax to completion, but the wick is still pretty warm. Some of the wax burns, but incompletely, forming soot. Some of the wax doesn't burn at all, and instead vaporizes. This wax vapor cools when it hits the air around it, forming … teeny tiny droplets. Perfect for making iridescence.

I'm also fascinated by the waves and streamers in the smoke. I think (and I'll note I'm guessing a bit here) that's from the warm air coming off the blown-out wick interacting with the cooler air from the side that blew it out. The cool air goes over and around the wick, and a fluid like air naturally sets up waves when it flows around an obstacle (you really want to click that and look at the photos and the video).

I will never blow out a candle and see it the same way again.

But that's the point, isn't it? Once you take something, no matter how mundane, and examine it carefully, look at the pieces of it, the forces, the motion, and really see it, you glimpse the rules that the Universe itself follows.

See? Science is beautiful. It's worth saying again.