Fig. 347.—Shadow when source of light is large.
355. Shadows.—A shadow is the space from which light is cut off by an opaque body. Thus if a book (see Fig. 346) is held between a screen, N, and a small source of light, L, a shadow is produced which extends from the book to the screen. Notice that the shadow is a space and not an area. If a large gas flame (see Fig. 347) is used as the source of light, the shadow of the book is no longer clear cut at the edges as before, but has a darker central part with a lighter fringe of partial shadow at the edges. The dark portion within the shadow has all the light excluded from it and is called the umbra. The lighter portion of the shadow at the edges has only a part of the light from the flame cut off. This portion is called the penumbra. when one stands in sunlight his shadow extends from his body to the ground or object on which the shadow falls. At night we are in the earth's shadow, which extends out into space beyond the earth.
Fig. 348.—Character of the earth's shadow.
356. Eclipses.—Since the sun is a very large object the shadow cast by the earth contains both umbra and penumbra. (See Fig. 348.) When the moon passes into the shadow of the earth, there is said to be an eclipse of the moon, while if the moon's shadow falls upon the earth, the portion of the earth cut off from the sun's light has an eclipse of the sun.
357. Images by Small Apertures.—The straight line movement of light makes possible the pin-hole camera, by which satisfactory photographs have been made. The action of this device may be illustrated by placing a luminous body, a lighted candle, an incandescent lamp, or a gas flame, in front of a piece of cardboard, S, which has a small opening in it. Light from the object (see Fig. 349) falls upon a screen, S2, so as to produce an inverted image. Other applications of this principle will be given later.
In Fig. 349 let PQ represent a gas flame, then light from point P at the top of the flame will pass in a straight line through the opening or aperture of the cardboard and strike at P2 at the bottom of the illuminated spot upon the screen. Light from Q passing in straight lines through the aperture will strike at Q2 at the top of the lighted space. This spot of light will have the same outlines as the luminous body PQ and being formed as just described will be inverted.
Fig. 349.—Image formed by a small aperture is inverted.