. The principles of physics. Experiment 4. — Repeat the above experiments, employing the smallerpiece of tin, and note all differences in phenomena that occur. Hold ahair in the sunlight, about a centimeter in front of a fly-leaf of this book, and observe the» shadow cast by theR — ^ hair. Then gradually increase the dis- tance between the hair and the leaf,and note the change of 227 ^^ ^^^ source of light were a single luminous point, as A (Fig. 227), theshadow of an opaque body B would be of infinite length, and would con-sist only of an umbra. But, if the source of light have
. The principles of physics. Experiment 4. — Repeat the above experiments, employing the smallerpiece of tin, and note all differences in phenomena that occur. Hold ahair in the sunlight, about a centimeter in front of a fly-leaf of this book, and observe the» shadow cast by theR — ^ hair. Then gradually increase the dis- tance between the hair and the leaf,and note the change of 227 ^^ ^^^ source of light were a single luminous point, as A (Fig. 227), theshadow of an opaque body B would be of infinite length, and would con-sist only of an umbra. But, if the source of light have a sensible size,the opaque body will intercept just as many separate pencils of light asthere are luminous points, and consequently will cast an equal number ofindependent shadows. Let A B (Fig. 228) represent a luminous body, and C D an opaquebody. The pencil from the luminous point A will be intercepted between. Fig. 228. the lines C F and D G, and the pencil vfrom B will be intercepted betweenthe lines C E and T> F. Hence, the light will be wholly excluded onlyfrom the space between the lines C F and D F, which enclose the enveloping penumbra, a section of which is included between thelines C E and C F, and between D F and D G, receives light from certainpoints of the luminous body, but not from all. LIGHT EEQTTIRES TIME TO PASS THROUGH SPACE. 325 Questions. 1. Why are images formed through apertures inverted ? 2. Why is the size of the image dependent on the distance of thescreen from the aperture ? 3. Why does an image become dimmer as it becomes larger ? 4. Why do we not imprint an image of our person on every object infront of which we stand ? 5. Upon what fact does a gunner rely in taking sight ? 6. Explain the umbra and penumbra cast by the opaque body H I,Fig. 228. 7. When will a transverse section of the umbra of an opaque body belarger than the object itself ? 8. When h
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectphysics, bookyear1895
