Rainbows are actually circular. We don't typically see a full circle rainbow because the Earth's horizon blocks the lower part.
Seeing a rainbow can feel like a reward. After a violent thunderstorm, it's nice to spot a colorful arch crossing the calming sky. But you might (or might not) be surprised to know that rainbows aren't really arches, nor are they "bows." They're actually full circles.
So why do we only see an arch? Oftentimes, the rainbows we see are partly blocked by the ground and horizon. To observe one in all its circular glory, you'd have to find a nice high vantage point. We'll explain how the phenomenon happens.
Mediums matter: In the air, light cruises along at 186,000 miles per second (300,000 kilometers per second). But since liquid water is denser, the 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 a mixture of all the colors within the visible light spectrum.
Each of these has a different wavelength; the longest belongs to red light while the shortest is 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.