Rainbow Rainbow Rainbow Rainbow · 2:42pm Apr 26th, 2015
Wow has it really been three weeks since I last did a blog post?
First news this week: Quillo Manar did a rather nice reading of The Art of Eclipse Engineering on YouTube, so you can now enjoy listening to this short story accompanied by appropriate music and pictures of eclipses. You don’t need to wear goggles.
Concerning my older, and similarly titled The Art of Rainbow Engineering, I note that Rainbows are suddenly being talked about this week following this photo by Amanda Curtis from New York, which took off on Twitter:
And Admiral Biscuit kindly posted this one below my story:
So, Rainbow fans, it is surely time to look beyond double rainbows, and ask the question: how can you get three or four rainbows visible at the same time from the same place. Without using magic of course—that would be cheating.
Usually when people claim to have seen a quadruple rainbow, it’s a double rainbow with the top cut off by clouds, which doesn’t really count.
As explained in the story, a primary rainbow is from sunlight reflected by raindrops at an angle of 42 degrees to the sun. A secondary rainbow can be formed from a double reflection in the droplets, giving a fainter inverted bow at 51 degrees. Can we keep going this way and reflect light three or four times inside a water drop?
Indeed we can. And with a sufficiently high power laser, these have been produced in a laboratory up to 200th order. This is fine to test the theory, but using a laser is kind of cheating, and as the laser light is just a single color, it’s not the same. [Observation of high-order rainbows formed by a pendant drop.]
But third and fourth order rainbows have been seen in the sky. They are much harder to see, being fainter than secondary rainbow as every extra reflection will diminish the intensity. More of an issue is that they appear on the same side of the sky as the sun. To search for them you need some sort of aperture to block out the sun, but they have been photographed, and with a bit of image processing you can make out the rainbows. [Photographic evidence for the third-order rainbow, Photographic observation of a natural fourth-order rainbow]
But the rainbows photographed by Amanda Curtis were a different sort. As can be seen from the photograph, there are two pairs of rainbows centered on different foci. Was this photo taken on a planet with two suns? Almost. One pair is the primary and secondary rainbow formed by the reflection of sunlight in the rain as usual. The other is produced by the sunlight reflected in a body of still water back into the sky, so it forms a rainbow centered on a different point. Such reflection rainbows are exceptionally rare as you need the perfect location with the sun, rain clouds, and a lake or other body of water, all in the right place. A challenge even for an experienced team of weather ponies.
I wonder if we could get 8 with a full moon at sunset
Fairly low probability, but possible?
Another interesting thing about rainbows is that their light is polarized. If you have a pair of polarized sunglasses, you can see this effect easily. The part of the bow aligned with the polarization filter will appear bright, as all of its light gets through. Ninety degrees from that, its light vanishes as it's fully blocked by the filter. Tilting your head will change which parts of the rainbow are visible and suppressed.
I was wondering how that worked. You might say that the reflecting body of water is...
A Mirror Pool.
3018520
Oh lord.