RainbowsRainbows result from refraction of sunlight in falling water droplets plus reflection of the light from the back of the droplet. If during a rain shower you can see the shadow of your own head, then you are in position to see a rainbow if conditions are favorable. The rainbow forms a circular arc around the anti-solar point, which is located at the shadow of your head. You can search for the shadow of your head to find a rainbow in a waterfall, or even in the spray from a hose or sprinkler. (Photo by Dubby Waggoner, used by permission.)
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Index Rainbow concepts Atmospheric optics concepts References Greenler Schaaf | ||||||
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Primary Rainbow
Rainbows are not seen in midday since the whole 42° circle is below the horizon at most latitudes. So rainbows tend to be seen most in the later afternoon when a thundershower has passed and the sun from the west is illuminating the receding edge of an eastwardly moving raincloud. It is possible to see the entire circle of the rainbow from an airplane since there can be falling droplets both above and below you.
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Index Rainbow concepts Atmospheric optics concepts References Greenler Schaaf | ||||
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Secondary RainbowThe secondary rainbow is about 10° further out from the antisolar point than the primary bow, is about twice as wide, and has its colors reversed. According to Schaaf, the light of the secondary bow is one-tenth the intensity of that of the primary bow, given the same viewing conditions. Photos by Dubby Waggoner, used by permission. Note the reversal in the order of the colors in the secondary rainbow. These photos also show the increase in brightness inside the primary rainbow. The region between the two rainbows should be a bit darker than the sky just outside the secondary rainbow, but this is a smaller difference.
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Higher Order BowsThe third order bow has an arc of 40°20' around the sun itself, so you would have ot look back toward the sun to see it. Schaaf reports that it has been seen a number of times in nature, but that no record of the sighting of the fourth order bow is found. A sighting of the fifth order bow is attributed to nineteenth-century scientist Eleuthere Mascart. It would be positioned between the primary and secondary rainbows and would be about 7° wide, its red overlapping that of the primary bow. Rainbow orders up to thirteen have been seen with a laser and water droplets in the laboratory. Experiments for the observation of higher order bows are described by Jearl Walker in "The Amateur Scientist" section of the July 1977 issue of Scientific American. |
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