Buzz
Most rainbows we observe appear as arcs because the ground and horizon partially obstruct the view. In fact, every rainbow is a complete circle, like this one captured over Gorssel, Netherlands. Photo by Martijn Harleman/SpaceGallerySpotting a rainbow can feel like a gift. After a fierce thunderstorm, it's a delight to see a vibrant arc stretching across the sky. But you might be surprised to learn that rainbows aren't really arcs or 'bows.' They are, in fact, full circles.
So why do we only see an arc? Often, the rainbow's circular form is partially blocked by the ground and horizon. To witness the full circle, you'd need a high vantage point. Here's an explanation of how this natural phenomenon occurs.
Degrees of Separation
The medium matters: In air, light travels at 186,000 miles per second (300,000 kilometers per second). However, in liquid water, which is denser, light moves much slower. So when a light beam, zipping through air, enters water, it slows down considerably.
For rainbows, sunlight enters each individual droplet and bends, or refracts, several times. First, it bends when it enters the water droplet. Then, it bounces off the far side's inner wall before reemerging into the air. As it exits, the light refracts once again.
As a result of refraction, the droplets split sunlight into its individual colors. While sunlight appears white to our eyes, it's actually a mixture of all the hues in the visible light spectrum.
Each color has a distinct wavelength. Red light has the longest, while purple has the shortest. Due to these differences, when sunlight enters a droplet, each color refracts and exits at a different angle, causing the colors in a rainbow to separate into distinct bands.
To catch a glimpse of a rainbow, you need to look away from the sun and have a high concentration of water droplets in the air in front of you. When a beam of white light hits these droplets, the light's colors scatter.
Rain O's
The colors you see depend on your position. Each water droplet in the mist acts as a tiny prism. They all separate white light into distinct beams of red, orange, yellow, green, blue, indigo, and purple. However, you’ll only ever see one color per droplet (if that), because all other colors exit the droplet at angles that don’t align with your eyes.
Purple sits at the bottom of a rainbow because it exits water droplets at the sharpest angle: 40 degrees from where it entered. On the other hand, red, at the top of the rainbow, is reflected back towards you at a 42-degree angle.
A crucial element in the rainbow’s formation is the antisolar point. This is the location in the sky (or on the ground) directly opposite the sun, 180 degrees away from your perspective. On a sunny day, the tip of your shadow points directly to the antisolar point. Every rainbow forms a perfect circle centered around this point.
If you're standing at ground level, you won't be able to see the bottom half of a rainbow's full circle. Any part of the rainbow that dips below the horizon becomes hidden. This happens because the Earth's surface is so close that it restricts the number of raindrops within your line of sight.
The portion of the rainbow visible to most people depends on the sun's position. When the sun is just starting to peek over the horizon, the antisolar point rises higher, allowing you to see a larger rainbow than when the sun climbs even further into the sky.
However, if the sun is more than 42 degrees above the horizon, no part of the rainbow will be visible from the ground. But from the skies, things get more exciting. On rainy or misty days, passengers and pilots sometimes get a chance to witness full circular rainbows. In 2013, photographer Colin Leonhardt captured this image of a circular double rainbow over Australia’s Cottesloe Beach while in flight.
Glory Be
To wrap up, let’s look at some round, colorful optical phenomena that may appear similar to rainbows, but aren’t. On your next flight, grab a window seat, and you might spot a glory. These are small, tight circles that form around the antisolar point when you look down at clouds or fog. Unlike rainbows, they're created by electromagnetic waves, and you can sometimes see them from high mountain peaks.
Another fascinating sight is the multicolored halos that sometimes form around the sun or moon. These mysterious rings are created when light interacts with suspended ice crystals in the atmosphere.
A bridge in Jim Henson's childhood town of Leland, Mississippi, has been officially named "The Rainbow Connection Bridge" to honor him. (For those who don’t know, "Rainbow Connection" is an Oscar-nominated song performed by Kermit the Frog — Henson's beloved character — in the 1979 film "The Muppet Movie".)
