There’s a very clear rainbow over town today, and I got to wondering. When a rainbow forms, what determines its size and thickness? Also, why are there double, and sometimes triple, rainbows? Are they always there, but just hard to see at times?
It’s mostly angles and the distance between you and the raindrops that are causing the rain to form.
Wowsers. Thanks!
The angular radius of all primary rainbows is the same, and is determined by the index of refraction of water. Secondary rainbows likewise all have the same angular radius (larger than that of the primary, of course). And I’d be willing to bet that you’ve never actually seen a tertiary rainbow: They can just barely be detected by specialized equipment, under perfect conditions, and even then only if you know exactly where to look.
Would swear I have as a kid, but can’t back it up with anything. 
**What determines a raindow’s radius? **
The distance between you and a pot of gold.
I have, but it was a special case and looked like this (not my picture).
Note how the middle rainbow is not concentric with the other two - that’s because it’s actually centred around the reflection of the sun in a body of water (lake or sea).
More examples on this page - the second photo is amazing.
Me too.
A reflected rainbow is not the same thing as a tertiary rainbow. They’re rare, though as documented in those pictures, they do occasionally happen and get observed.
Although, for that matter, I suspect that most of those pictures on that site aren’t true reflected rainbows, either: The angles don’t seem to match up. A true reflected rainbow, formed by reflection from the surface of a body of water, should have the reflected bow and the primary intersecting exactly at the horizon, and with the angles at which they meet the horizon being equal (the picture from Spitsbergen, about 60% of the way down the page, illustrates this beautifully). Most of those pictures, though, don’t seem to match that arrangement. At a wild guess, I’d say that they’re from a combination of a fata morgana mirage effect producing an extra image of the Sun, rather than from a reflection.