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Post by jonjel on Sept 28, 2016 14:36:09 GMT 1
I could probably look this up, but will display my ignorance here.
I took a picture of a double rainbow a couple of months ago.
But when I take a close look the colours on the outer rainbow are reversed. They are a mirror image of the 'primary' or innermost rainbow and do not follow the normal laws governing the wavelengths of the light being refracted.
Can someone explain why?
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Post by jonjel on Oct 3, 2016 10:35:54 GMT 1
Ah well. Seems I will have to look this up!
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Post by Progenitor A on Oct 3, 2016 13:38:33 GMT 1
Ah well. Seems I will have to look this up!  Let me know Jonjel There is a well-known phenomenon called temperature inversion where that is a distinct line separating two layers of atmosphere of different densities. This cause 'funnelling' of radio wave so that in the summer (when these things occur mostly)Froggy TV stations interfere with ours. Perhaps it has something to do with that and the lesser density layer is refracting in the opposite direction to the higher density layer and the angle of light incidence is just right to intercept that demarcation line Then again, perhaps not |
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Post by jonjel on Oct 3, 2016 15:06:19 GMT 1
Ah well. Seems I will have to look this up! Let me know Jonjel There is a well-known phenomenon called temperature inversion where that is a distinct line separating two layers of atmosphere of different densities. This cause 'funnelling' of radio wave so that in the summer (when these things occur mostly)Froggy TV stations interfere with ours. Perhaps it has something to do with that and the lesser density layer is refracting in the opposite direction to the higher density layer and the angle of light incidence is just right to intercept that demarcation line Then again, perhaps not I will look it up PA, but it was interesting to the curious(or feeble) minded. |
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Post by alancalverd on Oct 3, 2016 22:34:49 GMT 1
It's nothing to do with temperature inversion, but I can't remember how to explain it from first principles! Best try Wikipedia, but do come back if that doesn't help and I'll have another trawl through the optics textbooks.
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Post by jonjel on Oct 4, 2016 15:52:31 GMT 1
Well, I found this. I think I understand it but will read it a few times. Then I can bore the grandchildren!
Secondary rainbows are caused by a double reflection of sunlight inside the raindrops, and are centred on the sun itself. They are about 127° (violet) to 130° (red) wide. Since this is more than 90°, they are seen on the same side of the sky as the primary rainbow, about 10° above it at apparent angles of 50–53°. As a result of the "inside" of the secondary bow being "up" to the observer, the colours appear reversed compared to the primary bow. The secondary rainbow is fainter than the primary because more light escapes from two reflections compared to one and because the rainbow itself is spread over a greater area of the sky. Each rainbow reflects white light inside its coloured bands, but that is "down" for the primary and "up" for the secondary.[25] The dark area of unlit sky lying between the primary and secondary bows is called Alexander's band, after Alexander of Aphrodisias who first described it.[26]
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