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Rainbows - Page 2

© Keith C. Heidorn

When sunlight from very near the horizon produces a rainbow, it may not contain the full spectrum of colours when it strikes the raindrops. Often, such light has lost its bluer wavelengths, giving the sun a reddish appearance. If light rays entering a raindrop are primarily of one colour, the resulting rainbow will reflect that colour. Red incident light beams produce reddish rainbows. Similarly, if the sunlight is orange or golden, so too will be any resulting rainbow. I witnessed a very stunning orange-gold rainbow one December evening that was spectacular to behold, perhaps the single most beautiful skyscape I have ever beheld.

The afore-mentioned refraction-reflection-refraction process forms a primary rainbow, so-called because it is the product of a single reflection of light within the raindrops. Not all the returning light escapes the drop, however; some rays are internally reflected within the raindrop a second time before leaving the drop at a different angle (around 51 degrees from the original solar beam) than the single-reflection rays. The resulting rainbow lies outside the primary rainbow arch and is called the secondary rainbow since it forms from that second reflection. The secondary bow is weaker in brightness by about 43 percent than the primary bow, is twice as wide, and has its colour sequence in reverse order.

The combination of primary and secondary rainbows is called a "double rainbow." The secondary rainbow lies outside (above) the main primary rainbow. In the primary, violet/blue is on the inner edge with red on the outer. In the secondary bow, the red is on the inner edge and violet/blue on the outer. Between the two bows we see a much darker region known as Alexander's dark band. Most of the returning light from this region has been bent out of our view, thus leaving this section of sky depleted of light rays. In contrast, the rainbow centre appears much brighter than the rest of the sky because there the sunlight reflects directly back from the drop surfaces to our eyes.

Every rainbow forms a circle. How much of the circle we see depends on several factors including the width of the rain band and the position of the sun relative to our eyes. The centre of the circle can always be located by imagining a line from the solar disk through our eye position to the sky before us, the antisolar point. As the solar position rises in the sky, the antisolar point lowers until the rainbow arch sinks fully below the horizon. That is why midday rainbows are uncommon to ground observers. However, a rainbow can be seen around midday

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