No light without shadow

A shadow is a volume of space not directly illuminated when light is intercepted by an object. Usually we are aware of these shafts of darkness only when they fall upon an illuminated surface, where they are seen as dim, distorted outlines. But if the medium through which the shaft of the shadow passes is filled with particles able to scatter light, such as dust-laden or hazy air, fog, or turbid water, the shaft itself becomes visible.

We are apt to overlook the effect of shadows on the way the world looks to us. If we notice them at all, it is probably as a diversion.