I know the fall leaves steal the show, but the sky can be just as colorful day or night. This morning we got swiped by a wind of particles from a recent coronal mass ejection (CME) from the sun. Those living in the northern U.S.and Canada who were fortunate enough to be up after midnight probably saw the subtle pinks and greens of northern lights. If you’re like me and missed it, more of the same is forecast for tonight.
Care for more color? Yesterday afternoon a series of lenticular clouds tinged with the vivid pastels of iridescence paraded near the sun. You wouldn’t have noticed the colors at a casual glance because of the sun’s glare, but blocked by a building or tree, the clouds’ edges glowed in soap bubble pinks, yellows and greens.
Iridescence is seen in many places – peacock feathers, seashells, beetles, CDs, oil on pavement and yes, soap bubbles. Despite the intensity of the colors it produces, iridescence has nothing to do with pigments with a property of light called diffraction. It’s similar to the way a rainbow is pure light and not an object.
Recall that light is a series of waves much like waves made by boats motoring across a lake. Let’s imagine waves from those boats spreading until they come in contact. Some will complement each other to produce higher wave crests. This is called constructive interference; others will cancel each other out (destructive interference) to create flat water.
Extremely tiny, uniform-sized water droplets or ice crystals in certain types of clouds scatter light in all directions. The scattered light waves pass through one another just like the waves from our boats. Where crests coincide, that particular color or wavelength of light is reinforced and appears bright; where troughs coincide, the waves cancel or interfere with one another and that color is weakened or missing. Since light is composed of every color of the rainbow, a series of vivid, alternating hues results and tints the clouds’ edge.
Large water drops won’t do. Only tiny ones diffract light well. In particular, the edges of clouds are best because they’re exceedingly thin. Light passes through a single layer of droplets and streams to your eyes without being absorbed by intervening droplets. Many clouds start with droplets of uniform size but they either grow too big, become mixed with larger droplets or are simply too far from the sun to show iridescence. That’s why the sight is uncommon.
Iridescence happens in other ways. In soap bubbles, some of the light reflects off the outer layer while a portion continues through it and reflects off the layer’s inner surface. The second reflection is delayed by the extra time it took to penetrate the bubble and reflect back.
When the two rays rejoin, the crests and troughs aren’t lined up anymore. Scientists say they’re “out of phase” with each other. Some of the waves of each individual color of the spectrum reinforce each other; others interfere, creating delightful swirls of pastel colors.
Beetles employ a similar ruse. A thin, hard film on the surface of creature acts like the soap bubble film. Light reflecting from the interior is out of phase with that of the exterior, interference follows and we see striking colors as a result. Some insects and animals have a dark backing of melanin below the film to intensify the colors even more.
Wait a minute, didn’t we start out talking about the aurora? The places light will take us.