III. Absorption



The mechanism of the absorption of light by substances to produce color is obscure. It is apparently a function of the molecular structure of the substance. In the case of organic compounds, only unsaturated compounds show color, and their hue can be changed by altering the compounds chemically. Inorganic compounds are generally colorless in solution or liquid form, except for compounds of the so-called transition elements.

Color is also produced in other ways than by absorption. The colors of mother-of-pearl and of soap bubbles are caused by interference. Some crystals show different colors when light is passed through them at different angles, a phenomenon known as pleochroism. A number of substances show different colors by transmitted and reflected light. For example, a very thin sheet of gold appears green by transmitted light. The "fire" of certain gems, notably the diamond, is due to the dispersion of white light into its component spectral hues, as in a prism. Some substances, when illuminated by light of one hue, absorb this light and reradiate light of a different hue, always of longer wavelength. This phenomenon is called fluorescence, or, if delayed, phosphorescence. The blue of the sky is caused by the scattering of the short wavelength blue components of white sunlight by tiny particles suspended in the atmosphere. A similar scattering can be observed in a darkened movie theater. Seen from the side, the light beam from the projector appears blue, because of the smoke and dust in the air; yet the light on the screen is white. See also  Optics