If Rainbows Are Circular, Why Do We Only See Arches?

Mediums matter: In air,light cruises alongat 186,000 miles per second (300,000 kilometers per second). But since liquid water is denser, light can't move through it as quickly. So once a beam of light that's been zipping through the air hits a body of water, it slows down quite a bit.

In the case of rainbows, sunlight that enters individual water droplets bends — or refracts — multiple times. First, it bends upon passing into a bead of H2O. After that, the light bounces off the inside wall at the far side of the droplet and reenters the air. The light gets refracted again while exiting.

Through refraction, the droplets separate sunlight into its component colors. Although it looks white, rays of sunshine are in fact amixtureof all the colors within thevisible light spectrum.

Each of these has a different wavelength; the longest belongs to red light while theshortestis reserved for purple light. Because of those idiosyncrasies, when a beam of white sunshine enters a water droplet, its component colors refract — and exit the H2O — at different angles. That's why all the colors in a rainbow are divided into separate layers.

If you're hoping to witness a rainbow, your eyes must be pointed away from the sun — and there needs to be a large concentration of airborne water droplets in front of you. Once a beam of white light hits these, its component colors disperse.

Your ability to perceive some of those colors is dictated by your physical whereabouts. Each drop in that curtain of liquid water is a tiny prism. They all break white light into distinct beams of red, orange, yellow, green, blue, indigo and purple light. But your eyeballs will never see more thanone color per drop(if that). All the others exit the drop at the wrong angle to meet your pupils.

Purple is the lowest color on a rainbow because purple light exits water beads at the sharpest angle: 40 degrees relative to itsentry point. Meanwhile, red light — which sits at the top of a rainbow — gets sent back in your general direction at a 42-degree angle.

A key factor here is the location of theantisolar point. This is the spot in the sky — or on the ground — that's exactly 180 degrees away from the sun relative to your perspective. On a bright, sunny day, the head of your shadow marks the antisolar point. Every rainbow is a perfectly circular ringcenteredaround this very spot.

Yet if you're standing at ground level, you won't be able to see the circle's lower half. Indeed, from this vantage point, basically any part of a rainbow that dips below the horizon is rendered invisible. One of the reasons for this is that the close proximity of Earth's surfacelimitsthe amount and concentration of raindrops within your line of sight.

As such, the percentage of a rainbow that's visible to most people is directly correlated with the sun's position. When our solar neighbor is just barelypeeking over the horizon, the antisolar point will be fairly high up, affording you the chance to see a much bigger rainbow than you would when the sun climbs higher.

Conversely, if the sun is more than42 degrees以上horizon, it becomes impossible for ground-based observers to see any portion of a rainbow whatsoever. But when you're soaring in an aircraft, things get more interesting. On rainy or misty days, airplane passengers and pilots occasionally seefull circular rainbows. Better yet, in 2013, photographer Colin Leonhardt capturedthis pictureof a circulardoublerainbow while flying around Australia's Cottesloe Beach.

We'll close by talking about some round and colorful optical illusions thatlooklike rainbows but aren't. Book a window seat on your next flight and you could bear witness to aglory. These are tight circles that can appear around the antisolar point when you're staring downward at a cloud or a blanket of fog. Unlike rainbows, they're the product ofelectromagnetic waves. It's possible to see glories from tall mountain peaks.

Also noteworthy are the multicoloredhalosthat sometimes encircle the sun or moon. The eerie rings are made by the interaction of light and suspended ice crystals.