This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1909 Excerpt: ...screen and the hole in the hood are all in the same straight line. This shows that light is propagated in straight lines. Certain phenomena which cannot be taken up in an elementary book show that this statement is not strictly true. The straight lines along which light is transmitted are called Rays. When the source ...
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This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1909 Excerpt: ...screen and the hole in the hood are all in the same straight line. This shows that light is propagated in straight lines. Certain phenomena which cannot be taken up in an elementary book show that this statement is not strictly true. The straight lines along which light is transmitted are called Rays. When the source of light is at a great distance the rays are sensibly parallel. A bundle of parallel rays is known as a Beam. Rays of light which diverge from a point are called a Divergent Pencil, and rays which converge toward a point are known as a Convergent Pencil. 136. Shadows. The interposition of an opaque body in the path of a pencil of rays cuts off the illumination from the space behind the body. This unilluminated area is called the Shadow. The rectilinear propagation of light is the cause of shadows. Two kinds of shadow are distinguished: the Umbra and the Penumbra. When the luminous source is a point only, as in Fig. 48, the shadow is an umbra. When the source of light has finite dimensions, as in Fig. 49, the shadow is made up of two parts: the umbra U, and the penumbra P. When the opaque body is smaller than the luminous one, the umbra will converge to a point. When the opaque body is the larger, the umbra will be as shown in Fig. 50. There is no sharp line of demarcation between the umbra and the penumbra. 137. Formation of Images. An image is a picture of an object formed by rays of light coming from it. In Fig. 51. Fig. 51, we have an image of the candle produced on the wall of the box by means of the small aperture. It will be noticed that the image is inverted, this being true of all images formed in this way. This is the principle of the so-called pinhole camera. Experiment 67. Support two large sheets of cardboard about 15 cm. apart and ...
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