http://www.straightdope.com/mailbag/mdoublerainbows.html
Good report, Karen. Rainbows are important to me. I’ve been giving optics lectures on them since grad school. You covered it pretty well, but I’d like to emphasize a couple of points:
1.) The tertiary and quarternary rainbows can be formed (as can higher orders), but you won’t see them in nature. Both the third order and second order rainbows are relatively close to the sun, rather than awauy from the sun, so the conditions you would need to be able to see them – the sin breaking through a dark cloud that wiould provide the raindrops and the dark background, for instance – virtually never happen, anfd the very low intensity of the bow combined with glare from the sun would probably make it impossible even if they did occur. The tertiary rainbow WAS seen in the laboratory in the 14th century by al Farisi and al Shirazi.
2.) Diffraction always occurs, and is responsible for part of the way the rainbow looks. Without diffraction, the red edge of the rainbow would be sharply defined for each color, rather than gradually tailing off as it does. The relative widths of the colors vary with raindrop size because of diffraction. I’ve never heard of rainbows with red on the inside, but with very small drops (“Fogbows”) the rainbow itself is white, or it might even have a red edge on the inside. Diffraction is also responsible for what you seem to be referring to when you say “you might get smaller rainbows inside the main bow” . These are the supernumerary rainbows that stumped early theorists of the rainbow. They have the appearance of alternating bands of aqua and purplish pink inside the violet portion of the main bow, but aren’t always present.
Good references on this stuff are Greenler’s book on Rainbows, Minnaert’s book on Light and Color in the Open Air, and (if you want the math) R.a.R. Tricker’s Introduction to Meteorological Optics, which is a pretty good intro, taking the math in small steps.
I never saw a white rainbow until this December, while I was in Kauai. It gradually evolved into a very bright, normal rainbow. I was overlooking one of the valleys on the Na Pali coast from the top. It was raining for almost the entire 8 mile hike, but the cloud moved into the valley during a brief lull. I suppose I was looking into almost the top of the cloud, where the raindrops are smallest and not yet actual raindrops. Gradually the cloud moved out to sea, and become a normal rainstorm at a distance, allowing me to see the normal rainbow arcing through part of the valley.
Last summer I was camping up in the woods of northwest Ontario and woke up alone very early one morning to go fishing, just before sunrise. The sky had been pretty clear all night but as the sun was coming over the clear horizon, most of the rest of the sky was quickly filling up with dark rainclouds.
As I sat in my little rowboat in the middle of the lake I watched in awe at the brilliantly red sunrise – a darker, deeper, more vibrant red than I had ever seen in the surrounding clouds, while the sun was shining very brightly.
Then I looked over my shoulder at the dark western sky, and saw not one or two, but three rainbows. Even the tertiary one was bright enough so that I could almost follow it down to the horizon. It lasted for what seemed like a long time - it might have been five or ten minutes – until the sun disappeared behind the clouds and it began to rain.
It was one of the most magnificent sights of my life. Curses on me for not having my camera!!
You said you won’t see Tertiary rainbows in nature, but I’ve seen them quite often. In Hawaii we get them every now and then, but it seems different from what you’ve described. Rather than stacking on each other from faint to bright, the middle rainbow semed brightest to me, and the top and bottom rainbow about equally light. At the time I didn’t notice the color sequence. I just thought it was neat to see three rainbows. Is this a different from what you were posting about?
Also, the original article said that full circle rainbows were only visible from planes. However when the moon was particularly bright I’ve seen a rainbow encircling the moon before, while I was on land and not in a plane. It wasn’t nearly as bright as a normal rainbow, but it was clearly there.
I’ve seen all of these things while in Hawaii, and I’m not sure if the angle of light may be a factor or not.
I’d love to see that, Skammer. But I think that what you’re calling a tertuary rainbow wasn’t what’s usually called a tertiary rainbow (that is, a rainbow due to three internal reflections inside the drop). If it’s near the other rainbows, it’s in the wrong place for one. What you saw, considering that you were in a rowboat on a lake, was probably a reflected rainbow. There are two types of reflected rainbow, and the second type (less frequently seen, because so few people are lucky enough to be in the correct circumstances) is a rainbow caused by sunlight that reflects from the surface of the lake, then goes to the raindrops, and gets sent back to your eye. It was probably a reflected rainbow of the prinary sort. Such a rainbow is of the same diameter as the ordinary primary, but its center is higher in the sky (If you were to extend both the reflected rainbow and the primary to the horizon, you’d find that their edges meet, and that the center of the reflected rainbow is as far above the horizon as that of the primary is below the horizon). but people tend to perceive it as large in radius, especially if not the entire bow is visible.
lucky dog. I’ve never seen one.
(The first type of refected rainbow is a rainbow seen in the lake, as if it lies underwater. In this case you’re seeing light that goes from the sun, to the drops, gets refracted, reflected, reftracted, then it bounces off the surface of the lake and goes to your eyes. For more on both types, see Minnaert’s book, and especially Tricker’s, cited in my OP)
On the tertiary rainbow, see my comments in the last post.
“a rainbow encircling the moon” sounds like a lunar halo, not a rainbow. (The center of a rainbow lies on the point of the sky directly opposite the sun or moon, and is therefore underground unless you’re on the edge of a cliff/mountain/in a plane. And even then the angular radius of that bow is 42 degrees, centered on the antisolar (lunar) point, and wouldn’t seem to be circling the moon at all.
That could be the case. I don’t have any recollection of any of the rainbows being spectrum-reversed, either, so I can’t say for sure if the diameters were the same or not. Damn it – why didn’t I bring my camera?
I didn’t say that anything observed were rainbows near the sun. I said that tertiary and quaternary rainbows were near the sun – and they’re theoretically demonstrable to be there. al Farisi and ai Shirazi saw a tertiary rainbow in the lab (in the 14th century), using a glass flask full of water as a model raindrop. That wouldn’t produce parhelia, which are created by hexagonal ice crystals.
And nobody has reported seeing anything near the sun or moon except Staver, who reported the “lunar rainbow” near the moon, and which I suggested was more likely a halo. The 22 degree halo is produced by ice cystals.
Parhelia (and parselenes) are also produced by hexagonal ice crystals, but ones oriented preferentially by their fall through the atmosphere. By definition, these features do not and cannot encircle the sun or moon – they sit horizontally even with the body. For the right size range, they can have brilliant rainbow-like colors. But they don’t encircle.
Nope. Seen pictures of fogbows, but not the bows themselves.
Lunar fogbows have two things working in favor of white – fogbows themselves are white or only lightly colored, since the small doplet size produces very broad bows that overlap for all colors, so there’s little or no separation.
Plus, they’re at night, when the lighting is low and you’re mainly seeing by rods, which have no color perception.
I saw a triple last summer in Seattle. It was early evening, with a quick sunbreak after a storm. The order of the three bows (from inside to outside) was “very dim”, “blindingly bright”, “dim”. All three were superimposed against very dark storm clouds, which is probably why I could see the dim one at all.
It did not appear to be a reflection rainbow like on the referenced atmospheric optics website, all three bows appeared to be concentric, but I could only see about 30 ~ 45 degrees of the bows so I’m not sure. I am in between two large bodies of water though, so it would certainly suggest a reflective third since a true third refractive bow is so rare.
Rainbows are not an optical illusion. And optical illusion is some anomaly in human perception. That the full moon appears huge on the horizon compared to when directly overhead is an optical illusion,
Rainbows are “real,” at least as real as blue sky. Its position is dependent on the viewer, but so what. Those photons hitting the back of your eye are real.
I have to admit that I haven’t seen such a configuration (although I’d love to), but it’d be hard to explain a third truly concentric bow. Reflected rainbows can be very confusing. A “rainbow pillar” that seems to be a straight, straight-up vertical rainbow is often reported to be issuing from the point where a rainbow encounters the ground. Such a thing is almost certainly due to a reflected rainbow, where only the portion close to the ground is seen (probably because that’s the only place where the raindrops are that are needed to make the bow – fragmentary rainnbows are more common than complete ones). If it is a reflected bow, it will be curved, with the same radius as the main bow. But because only a portion is seen, and because of its unusual orientation (it will have to bow OUT before curving back in towards the center), it’s perceived as straight.
I have. It took me a moment to figure out what it was because I’d never thought of such a thing before. I was hiking the Coyote Hills, a small group of hills on the eastern side of the southern portion of San Francisco Bay. It was about an hour before sunset. There was both a double direct rainbow and the rainbow from the reflection off the bay.
Another intersting rainbow phenomenon is supernumerary arcs (rare, but I’ve seen these a few times). If you scroll down a ways on this Wiki page you’ll see a decent photo and explanation.
