WRAL WeatherCenter Blog

Seeing Double!

Posted May 22, 2006

Rainbows, that is... below are two very nice photos of well-formed rainbows sent to us by Bill Dinwiddie of Garner. He shot these pictures on US 40 the afternoon of May 18th around 5:30 PM. You can tell that it wasn't very late in the afternoon (relative to sunset) because the arc of the rainbows is fairly shallow, indicating that the sun is still a bit high in the sky. This is because the rainbow is essentially part of a circle, the center of which is located at the "anti-solar" point formed by an imaginary line from the sun through the back of an observer's head and onward toward the front. When the sun is about to set, the center of the rainbow is almost at horizon level and the bow will form almost a half-circle, rather than the upper part of the circle you see below.

There are several neat things to observe in these pictures. First of course is the "primary" rainbow, formed by two refractions of sunlight entering and leaving, and one internal reflection inside, raindrops. This is the bright bow that follows the color pattern ROY G BIV, with Red on the outside and Violet on the inside, at a 42 degree angle from the anti-solar point. There is also a fainter "secondary" bow visible in these photos, formed by two internal reflections and located about 51 degrees from the center. The second reflection diminishes the intensity of the light passing through the drops, and thus the second bow is not as bright, and is occasionally too faint to see. In addition, the second reflection causes the outer bow to be a mirror image of the inner one, so the colors are reversed (with red on the inside). Third and fourth order bows, involving even more reflections within the droplets, are possible in theory but are rarely if ever seen in nature.

They are a little difficult to see in these photos at the reduced size, but just inside the primary bow, especially toward the left and right ends, one can also make out faint "supernumerary" arcs, which are interference phenomemna formed a little differently from the others, and usually involve a series of pink and green color bands that are clearly visible only with the most intense bows.

Finally, a phenomenon that often goes unnoticed but is clearly visible in Bill's images is "Alexander's Dark Band," the region in between the primary and secondary bows. Within that angle of your vision, almost no light is being reflected back toward your eyes by raindrops. Inside the primary and outside the secondary bow, drops do reflect some light in your direction, making the sky appear brighter there. This is related to a concept called "angle of minimum deviation". A good illustration of how this works is available at hyperphysics.phy-astr.gsu.edu/hbase/atmos/ligsky.html. Thanks again to Bill for the nice shots, and keep your cameras handy everyone!
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