The rainbow is best observed when the sun is not more than 45 degrees above the horizon; it forms in the side of the sky opposite the sun. On rare occasions a tertiary bow may be seen between the observer and the sun.
The colors of the rainbow vary in intensity and in quality. Red is always in evidence outside the primary and inside the secondary bow; orange, yellow and green are commonly though faintly observable; blue is sometimes seen; but violet is rarely if ever observable. The strength and the sequence of the colors depends mainly on the size of the drops, but partly on their distance and the number of them.
Each observer sees his own rainbow, and each rainbow is
Refraction of light passing through rain-drops.
practically a series of hollow concentric cones, the vertex being at the eye of the observer. The rainbow moves forward, backward or sideways as the observer moves. A shower in one part of the sky and sunshine in another, the observer being between, are requisite for rainbow formation; and this condition, in most parts of the world, is confined to summer showers.
Mirage.—Owing to changes in temperature the density of the air varies almost constantly at different heights. Rays of light passing through air of varying density are bent differently with each change of density. An observer looking at a distant object sees the object with distorted outlines. An elliptical sun at sunset is very common; and sometimes one sees it with greatly distorted outlines.
