How do rainbows form?
The short answer to this question: Light bounces off of water droplets and is split into the colors that make up white light. That refracted light is bounced back to our eyes as a rainbow.
But let's dive deeper. First we need to understand what "light" is and how it is "split" to form colors.
The sun emits light of many wavelengths, all of which are on the electromagnetic spectrum. The full spectrum of light in our universe includes the highest wavelengths of gamma radiation to the lowest wavelengths of radio waves. The tiny portion of the EM spectrum that our eyes can detect, between wavelengths of 380-740 nanometers, is called visible light. Also referred to as white light, this is the light that gives us the view of the world around us.
In the vacuum of space, light travels at 3 x 108 meters per second, or the speed of light. Different materials (mediums) will slow light down, like our atmosphere, or glass, or water. When light moves through rain, it is slowed down and bent. This change in speed will reveal the different wavelengths of visible light, in the form of their colors. This is called refraction. A voila, the visible light spectrum is revealed to us as red, orange, yellow, green, blue, indigo, and violet.
What viewing conditions do you need?
In order to see a rainbow you need to be placed under a clear sky between the sun and a rain shower. This allows for sunlight to hit the raindrop and bend back to your eyes without any obstruction.
Why are rainbows curved?
When sunlight is refracted from raindrops, it's happening all across the horizon. Most of the refracted light is scattered in directions that we cannot see from our perspective. The refracted light we can see is bent by the raindrop at a 40°-42° angle. That angle between the sun, rain, and us forms a curvature in the sky, hence the rainbow. A low sun angle will lead to a high arced rainbow and a high sun will lead to a shallow arced rainbow. If you're in an airplane, the full 360° rainbow will be seen.
What makes a double rainbow?
The double rainbow is formed by a second reflection within the droplet, this time at an angle of 50°. This second reflection actually reverses the colors of the top rainbow! The second rainbow is dimmer because the light intensity is essentially "diluted" in the second reflection. The second reflection in the rain drop always occurs, but can only be seen in perfect conditions.
Make your own rainbow
Making a rainbow is easy now that you know what causes it. You need sunshine, a medium to bend light, and your eyes! Try bringing a spray bottle of water outside and turn your back to the sun. Then spritz the water into the air at different heights until you see the rainbow. You can also purchase a glass prism and set on the window sill or take it with you to create your own personal rainbow wherever you are.