Primary rainbow

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primary rainbow

A rainbow that is distinguished from other rainbows by its angular radius, color order, and brightness.

This bow is seen between about 40° and 42° from the antisolar point (shadow of the observer's head) or equivalently, between 140° and 138° from a light source (such as the sun). Reds are found to the outside of the bow (closest to the sun) with the blues to the inside. The primary bow is usually brighter than any of the other bows. The primary rainbow is certainly the most frequently noticed bow, but the purity and range of its colors fall a long way short of that assumed by the popular dictum: all the colors of the fall. Frequently accompanying the primary bow are the secondary bow (lying about 8° outside the primary bow) and the supernumerary bows (immediately inside the primary bow, and often confined to the upper portions of the arc). Infrequently seen are the reflection bows. A theory of the bow that approximates the behavior of light as a ray is able to account for the difference in position and color order of the primary and secondary bows. In this theory, the position of each bow is determined by the minimum angle of deviation of the light passing through a drop. The difference is that the light that forms the primary bow has undergone one internal reflection, while the light that forms the secondary bow has undergone two internal reflections. This is a useful approximation to reality, but it fails to capture many important features of observable bows.
Compare secondary rainbow.


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