What Made This Rainbow So Perfect?
Conditions have to be just right to be able to see what we saw
“Mom, it looks like it’s going to rain, we should not be taking the dog for a walk right now,” warned my 12-year-old daughter.
“It’s done raining,” I reassured. “Let’s just go. Get some fresh air.” She wasn’t really happy about it, but off we went.
We made it to a nearby park and it was beautiful: a peaceful pond with geese and ducks, green trees, the scent of damp earth.
Then, of course, it started raining. It was just cold sprinkles at first, but then it downpoured. Laughing and whooping, we sprinted and made it to the road just as it stopped. The sun was low in the sky but it brightened up a lot and quickly.
“Look for the rainbow!” called my daughter.
And there it was:
And what a rainbow! We had never seen such a neon rainbow. A double one, too. Briefly triple. Cars pulled over, and drivers got out to take pictures. We all stood in awe.
“So, what makes a rainbow?” I asked.
We knew it had something to do with light hitting water and being refracted, like in a prism. But what’s really going on, and why was this particular rainbow so freaking bright?
Soooo: Sunlight is made up of these energy particles called photons. Here’s the critical thing, though– photons have different wavelengths that move at different speeds. We can only perceive certain wavelengths, and the wavelengths we can see make up the entire spectrum of color:
When photons hit a raindrop, they move through it, but because water is denser than air, they slow down. The shorter wavelength photons (which make blue) travel faster than the longer wavelength photons (which make red).
Because they move at different speeds, the photons get split up by wavelength—aka refracted—within the raindrop, divided into the spectrum of the colors of the rainbow. This then gets reflected off of the other side of the raindrop back at us, which is what we see:
And what about a double rainbow?
Some photons pass through the water and keep going. Others are reflected again through the raindrop in the opposite direction, which makes a paler secondary rainbow, with the colors in the reverse order from the brighter primary rainbow. Crazy, right? Look again at this rainbow photo, and you see that it’s true:
But why was this rainbow so day-glo?
In order to see a rainbow, you have to have the sun behind you and the rain in front of you. The sun has to be at 42 degrees or lower relative to the horizon for us to see the rainbow at all. Any higher, and the reflection ends up on the ground (as in, not visible). The lower the sun is in the sky the more complete arc you will see.
The brighter the sun (meaning, the clearer the atmosphere) and the larger the raindrops, the more intense the colors. We apparently had perfect conditions—with the sun as low in the sky as possible and as bright as could be in the presence of plenty of chunky rain droplets—for a full, blazingly neon rainbow!
Follow Aha! for more amazing science behind life’s most intriguing, strange and unexpected questions, like: Why is the Sky Blue?
