Page:Handbook of Meteorology.djvu/132



A ray of light may pass through a solid, such as glass; a liquid, such as water; or a gas, such as oxygen, nitrogen or water vapor—that is, the air—without much apparent loss. Such substances are transparent. In passing through different substances the ray is likely to be bent out of its course, as is apparent when a stick is thrust obliquely into a body of water. The bending of the ray is called refraction. Or if it is turned back, as when it impinges upon a mirror, it is said to be reflected.

A ray of light impinging upon a piece of black cloth is said to be absorbed. If only a part of the ray is absorbed, the rest being reflected, the parts of the ray reflected produce the sensation of color.

If a ray of light is passed through a wedge-shaped prism the component parts of the ray are unequally bent or refracted, and reach the eye in a series of colors. Red is the least refracted; violet suffers the greatest refraction. A ray of white light, therefore, is not of a “bundle” of wave-lengths of the same magnitude, but a bundle of an infinite number of rays of different wave-lengths.

In passing by the edge of an opaque body, or in passing through a very narrow slit, a ray of light is deflected slightly, and alternate fringes of light and dark bands are produced. The deflection and interference constitute diffraction, and diffraction is also a factor in giving various color tints to the sky.

The various atmospheric optical effects of the sky are produced mainly by refraction, reflection, diffraction, and absorption of light by the constituents of the air. The color of the sky itself is due to the irregular scattering and dispersion of light as the sun’s rays glance from the gaseous molecules and minute dust motes of the air. The most common incidents of atmospheric optical phenomena are coronas, halos, rainbows and mirages.

Corona.—A corona consists of a ring—sometimes several rings—rarely more than 4 degrees of arc measurement in diameter, surrounding the sun or, more commonly, the moon. The corona is a case of diffraction, the deflection of rays passing by water droplets. The inner border of the ring is