Rainbow, arch of light exhibiting the spectrum colors in their
order, caused by drops of water falling through the air. It is seen usually in the sky opposite to the sun at the close of a shower and also in the spray of waterfalls. In the brightest or primary bow, often the only one seen, the colors are arranged with the red outside. Above the perfect bow is a secondary bow, in which the colors are arranged in reverse order; this bow is dimmer, because of a double reflection within the drops.

When the sunlight enters a raindrop it is refracted, or bent, by and reflected from the drop in such a way that the light appears as a spectrum of colors.

The colors can be seen, however, only when the angle of reflection between the sun, the drop of water, and the observer's line of vision is between 40° and 42°.

When the sun is low in the sky the rainbow appears relatively high; as the sun rises higher, the rainbow appears lower in the sky, maintaining the critical 40°- to 42°-angle. When the sun is more than 42° above the horizon no rainbow can be seen because the required angle passes over the head of the observer.