Rainbow physics for 5-year-olds
While rainbows are often regarded as mysterious, miraculous or even connected to magical beings, they’re actually the result of physics and can be wholly explained. However, my kindergartner was planning to explain this to her peers on her own, some additional simplification and abbreviation was necessary.
White light from the sun is a contains the seven visible colors in the spectrum: red, orange, yellow, green, blue, indigo and violet. Each of these colors has differing energy levels, with red being the lowest energy, and indigo being the highest.
Now when that light goes from one material to the next, such as from the air to a water droplet, it uses some energy and generally changes speed in doing so. Think of pushing a wagon from cement onto grass- the grass offers more resistance, and you slow down a little. Just as with the wagon, if you cross from one material to the next head-on, you just slow down a bit. But if you come at an angle, your leading edge (or wheel) gets pulled first, pulling your course to the side.
So if light enters a droplet at an angle, the direction of the light gets pulled or bent, and this asymmetric adjustment frays the white light, splitting it into the seven colors. That bent light, at the proper angle, bounces off the back of the water droplet, and gets bent further when exiting the droplet. The exiting light is then highlight stratified, allowing us to see a rainbow.
The angles are very important in all this. As such, you’ll only see a rainbow when the sun is behind you, as the light will be entering and exiting the droplets from the same side. In that sense, rainbows only exist to viewers on one side of the droplets.
So how was all this translated by my daughter? Something like “light goes into the droplets and gets bent and split up so that it shows the seven different colors.” Close enough.
Source: How do rainbows form?, Canon Science Lab