Series of concentric, coloured arcs that may be seen when light from a distant source—usually the Sun—falls on a collection of water drops such as in rain, spray, or fog. The coloured rays of the rainbow are caused by the refraction and internal reflection of light rays that enter the drop, each colour being bent through a slightly different angle. Hence, the combined colours are separated upon emerging from the drop. The most brilliant and most common rainbow is the so-called primary bow, which results from light that emerges from the drop after one internal reflection. The colours of the arc (from outside to inside) are red, orange, yellow, green, blue, indigo, and violet. Occasionally a less-intense secondary bow may be observed; it has its colour sequence reversed.

This entry comes from Encyclopædia Britannica Concise.
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