Sun. Rain. Sun. Rain.
Did you see the rainbows last week? Rainbows come from the play of sunlight in raindrops, so they can only occur when there’s a mix.
Water lets some light through, of course, making rain see-through; water reflects some light, making rain sparkle.
Water can also refract some light. Light hitting the raindrop square-on passes right through it, but light striking the surface of a raindrop at an angle refracts (is deflected or bent) obliquely into the inside of the raindrop.
Refraction separates white light into its different colors, as in a prism. White light is a mix of all colors; each color is different wavelength of the light. The blue/violet end of the spectrum has shorter wavelengths; the red end of the spectrum has longer wavelengths. The shorter wavelengths bend more when refracted than the longer wavelengths do.
Once refracted, light inside the raindrop may pass directly through the surface and out of the raindrop, or it may reflect (bounce) off the inner surface of the raindrop. After the first bounce inside, the light may refract again before it finally exits the raindrop. The second refraction expands the separation of wavelengths that the first refraction caused.
Under the right conditions, some sunlight shining on a wall of falling raindrops may be refracted, reflected, and refracted again to emerge as a spectrum that travels back in a specific angle (42 degrees) to the sun. That’s what we see as the rainbow.
The right conditions are bright sunlight shining on the water droplets at that 42 degree angle to where you’re standing, and water droplets that are both the right size and rather spherical: if the droplets are too large or too small, or if they’re too wobbly, a rainbow won’t form; if the sun’s not at the right angle, you wouldn’t see a rainbow that does form.
A rainbow is bow, or arc-shaped, because we stand in one point and see the light refracting at a specific angle from the sheet of falling drops.
We see a giant rainbow from tiny drops because we catch slightly different bits of each drop’s spectrum. The spectrum coming off a given raindrop is organized top to bottom in this order: blue/violet, blue, green, yellow, orange, red.
However, the primary rainbow we see is in reverse order, with red at the top/outside, followed by orange, yellow, green, blue, and blue-violet on the bottom/inside.
What’s going on there?
When you see a rainbow, you’re looking at the accumulated tiny colored sparkles of each minute droplet in a tall sheet of droplets. From wherever you’re standing, you see the bottom of the spectrum from the droplets at the top of the sheet and the top of the spectrum from the droplets at the bottom of the wall — so it looks backwards.
This effect is also the main reason rainbows “follow” us when we move and why each of sees our own, unique rainbow.
A secondary rainbow, formed when light reflects twice inside each raindrop, is larger and fainter (because less light makes the extra bounces) than the primary rainbow. The double reflection can only occur when the drops are fairly small and very spherical. The secondary rainbow shows the same series of spectral colors as a primary rainbow, but in reverse because of the extra bounce.
There are other kinds of bows, too. A moonbow may show up at night, formed by moonlight (which is reflected sunlight) refracting and reflecting around the droplets of fog or high-altitude cirrus clouds between us and the moon. Sometimes, though rarely, a cloud layer, the sun, and an airplane’s altitude may line up just right to give the passengers a view of a full-circle rainbow below.
Other bows are closer to home. If airborne droplets are very small, as in fine fogs, a white rainbow or “fogbow” may appear. And, of course, rainbows may form around waterfalls and lawn sprinklers—any place where sunlight bounces inside water droplets of the right size and shape.
And the pot of gold at the end of the rainbow?
That has to be somewhere in the rain shower!