So waking up this morning, as one does, I caught an image going by on the Twitters (and here in the main thread comments at LGF).
Leaving aside discussions of what really is a quadruple rainbow (from the point of view of science, it is not just 4 rainbows in a single camera image), I was confused by the difference in angles in the bows.
Rainbows form, of course, when the sun is refracted (and reflected internally!) by raindrops. For the primary rainbow, the light is only reflected once. For the secondary rainbow, it is reflected twice (that’s what the colors are reversed!). Of course, because it needs to be reflected internally twice in a raindrop that’s falling, double rainbows are typically fainter. (Also, you get a really cool dark band between the primary and the secondary bows, called Alexander’s Dark Band.)
The primary and secondary bow are going to be parallel arcs, because the sun and the rain are in the same place for them. That’s why this image intrigued me so much this morning - from the brightness and the color patterns, it’s pretty clear that what’s captured were two double rainbows. (Not, from science’s point of view, a quarduple rainbow - which has a specific meaning and has been caught on film.) I didn’t understand what could cause the difference in angles though.
Fortunately, if you dig around on the Internet, people have answers for just about anything. Behold the reflection bow:
The centres of reflection bows are at the same altitude as the sun - the anthelic point. This is the same distance above the horizon as the centres of normal bows are below it at the antisolar point.
The normal bow and its corresponding reflection bow intersect at the horizon.
Somewhere nearby, there must be a relatively calm body of water producing reflections - some very nice and strong ones! Reflection bows are typically the brightest when the sun is low, which would be why it makes sense for an early morning commuter to grab a photo of it.
Science is so cool.