Page:Elementary Text-book of Physics (Anthony, 1897).djvu/415

§ 328] in the least time, from which waves will proceed to that point in sensibly the same time, and, meeting in the same phase, combine to produce light. It is also true that values change slowly in the vicinity of a maximum, and there are cases where the path followed by the light is determined by the fact that the time is a maximum instead of a minimum.

327. Shadows.—An optical shadow is the space from which light is excluded by an opaque body. When the luminous source is a point, or very small, the boundary between the light and shadow is very sharp. When the luminous source is large, there is a portion of the space behind the opaque body, called the umbra, which is in deep shadow, and surrounding this is a space which is in shadow with reference to one portion of the luminous source while it is in the light with reference to another portion. The space from which light is only partially excluded is the penumbra. Fig. 96 shows the boundaries of the umbra and penumbra. It is evident that the light diminishes gradually from the outer boundary of the penumbra to the boundary of the umbra.

328. Images by Small Apertures.—If light from a single luminous point pass through a small hole of any form, and fall on a screen at some distance, it produces a luminous spot of the same form as the opening. Light from several points will produce several such spots. If the luminous source be a surface, the spots produced by the light from its several points will overlap each other and form an illuminated surface, which, if the source be large in comparison with the opening, will have the general form of the source, and will be inverted. The illuminated surface is an inverted image of the source. If a small opening be made in the window-shutter of a darkened room, images of external objects will be seen on the wall opposite. The smaller the opening, the more sharply defined, but the less brilliant, is the image.