Thread by @drskyskull

So I go to a movie and come out seeing that all sorts of people are explaining the phenomenon wrong. To me. An optical physicist.

https://twitter.com/drskyskull/status/1434275648885952516

So let's have a quick #OpticsLessonOfTheDay: there are a bunch of different ways that you can get rainbows, and they're all different!

The most familiar rainbow effect is the one that Isaac Newton studied using a prism, and also forms the cover of Pink Floyd's Dark Side of the Moon:

(I remember reading somewhere that the depiction of prism diffraction on Pink Floyd's cover is not quite accurate, but it's close enough and I'm too lazy to figure out what's wrong right now so don't ask)

A prism creates a rainbow because of dispersion. Dispersion is the term used to describe the fact that different colors of light travel at different speeds in matter. Since refraction depends on the speed of light, different colors bend in different directions...

... and this results in a rainbow pattern coming out of the prism. Actual rainbows that you see after a storm also occur because of refraction: the light you see is light that has reflected once inside water droplets, and each color travels a different path.

Double rainbows arise from light reflecting twice in a droplet. I've written a detailed blog post all about rainbows for more information: skullsinthestars.com/2013/05/13/all…

But you can also get rainbow patterns from *diffraction*: the spreading and scattering of light due to its wave properties. A good example is a diffraction grating, which is a periodic structure that causes different colors to be reflected/transmitted in very specific directions.

A compact disc, which is a bunch of pits arranged in essentially a periodic pattern, produces a lovely reflection rainbow when you look at the shiny side.

(Quickly ran about the house looking for an old CD to photograph because images online kinda suck. Incidentally, a traditional CD works best. A DVD or blu-ray has smaller pits that don't diffract quite as spectacularly.)

Another way you can get a rainbow is through wave *interference*. When you have a thin film of oil on water, for example, you get a rainbow. (Image via Wikipedia)

Basically, part of the light illuminating the film reflects at the top surface, and part reflects at the bottom surface. Different wavelengths (colors) end up constructively interfering (looking brighter) at different angles.

It would be a shame for me to not mention rainbows you get with stress-induced birefringence! When you put a piece of molded plastic between a pair of crossed polarizers, you see lovely colors in the plastic.

In the molding process, the plastic becomes anisotropic -- it becomes sensitive to the polarization of light, and different colors have different sensitivities. When you use crossed polarizers, you can see this sensitivity.

That brings us to the glory (image via wikipedia). This isn't a rainbow effect, as the cloud droplet size is much smaller than the size of rain droplets. In fact, the cloud droplets are small enough so the wave properties of light are important.

The best non-technical description I can give? The light waves, in essence, "creep" along the surface of the cloud droplets when they interact with them, and different colors scatter back at different angles, giving a rainbow pattern again.

The glory is so tricky to explain in non-technical terms that H.M. Nussenzveig wrote a paper asking "Does the glory have a simple explanation?" To give you some context: Nussenzveig is basically the world expert on the subject.

This is actually part of what is cool about the glory: it is a phenomenon that everyone is familiar with, arguably mundane, but involves physical processes that are quite complex! Seeing the glory is a glimpse into really deep physics.

Last little glory trivia: people on mountains can cast enormous shadows when the sun is low, called a Brocken specter; this specter often comes with a glory. (Pic via wikipedia again.)

I'll end this thread here, but just say: hopefully this shows you that "rainbows" can arise from more than just rain! /END