Note:  Do not rely on this information. It is very old.


Shadow. When light falls upon an opaque body, it cannot traverse the space behind that body, and hence a region of darkness is produced, or the body is said to cast a shadow. If the light came from an absolute point, a projection of the object would be cast upon any surface behind it, the form of the projection depending on the shape of the surface and its position with respect to the object and the light. Usually the light does not emanate from a point, but the source of light has measurable size; in this case each point of light forms its own shadow, and the final result is a number of overlapping shadows, the darkest region being that where most overlapping takes place, and the lightest where least overlapping occurs. A shadow looks darker or lighter accordong as much or little extraneous light is about, its depth being merely estimated by contrast, If the surface receiving the shadow be near the object, a deeper shadow is obtained than when it is far away, owing to the fact that the rays of light which would be primarily intercepted by the object can, by reflection from other surfaces, etc., find their way into the otherwise dark region, if space enough be allowed for this behaviour,