We were totally wowed by the beauty and coolness of the rainbow pics posted by our Facebook friends. All I can say is – scroll down and enjoy!
Rainbows are always half circles or part of a circle. Why a circle? What is the reason for a rainbow shape to be round? The answer has to do with the way that rainbows are made via sunlight and water droplets.
A rainbow is caused by sunlight hitting water droplets in the air. You typically see a rainbow in the sky opposite the sun after or during a rain shower. When light enters a raindrop, it’s refracted, or bent.
Then the light is reflected from the drop in such a way that the white light breaks into its separate wavelengths and you see a spectrum of colors.
The colors are due to what’s called the prism effect. The specific angle is called the rainbow angle, first described by Descartes in the year 1637.
Sunlight that shines into a raindrop leaves that raindrop at an angle of 138 degrees from the path that the light traveled before it entered the drop.
If the light left the water droplet at 180 degrees, it’d head straight back toward the sun. With a rainbow angle of 138 degrees, the light is traveling in a direction somewhat, but not directly, back toward the sun. That direction of travel by the light explains why you always see rainbows when the sun is behind you.
The sunlight emerges from many raindrops at once. The combined effect is a mosaic of light, spread out in an arc in the sky.
But that’s not the end of the story. When sky conditions and your vantage point are perfect, the rain and sun work together in this way to create a complete ring of light – a circle rainbow. You’ll never see this from Earth’s surface because your horizon gets in the way.
So why are rainbows round? Why, in nature, are rainbows never straight? A rainbow is always curved because rainbow colors only appear when water droplets refract light at a specific angle with respect to the direction of the sun.
That happens in such a way that it makes a circle around the point that’s directly opposite the sun.
But you can see a small complete circular rainbow in the spray from a garden hose if you get the angle just right.
By the way, sometimes people say they see full-circle rainbows from airplane windows, with the shadow of the airplane inside the circle. But those aren’t true rainbows. Instead, those are called glories. Read about glories here.
One last thing. The early Greeks had a more fanciful – but very beautiful – explanation for rainbows. They believed that Iris, the Greek goddess of the rainbow, could fly at the speed of the wind from one end of the Earth to the other. As she flew, she left an arc of colors trailing in her wake.
Bottom line: Rainbow photos from around the world from EarthSky friends. Thanks, everyone!