Shadows

Shadows, as ordinarily understood, are the result of the interception of rays of light by opaque or semi-opaque substances. Close inspection of any shadow, whether cast by the sun or by an artificial light, shows that its margin is not clear cut. This is chiefly due to the fact that the source of light has a finite size. There must consequently exist certain regions from which the source of light appears to be only partially screened. In these regions the shadow is partial; whereas in regions from which the source of light is wholly screened by the intercepting substance, the shadow is complete. Other forms of radiant energy may be intercepted, and corresponding shadows cast. Take, for example, sound. To appreciate the existence of sound shadows we must ourselves get within the shadow, that is, we must set ourselves so that a large obstacle intervenes between us and the source of sound. In such circumstances the intensity of the sound becomes very much diminished. The deadening effect of an ordinary wall upon a sound originating at the farther side is a familiar illustration. In electrical radiations shadows also may exist, and may be made evident by suitable means. It should be remembered, however, that a substance which is opaque to one kind of radiation is not necessarily opaque to another. A stone wall is opaque to light, but is not opaque to electrical radiations of a certain kind. Again, rock-salt and glass cast no very apparent light shadows; but with radiant heat glass casts a distinct shadow, while rock-salt casts little or none. See HEAT, and LIGHT; also ECLIPSES.