15. A test of 20 110-volt tantalum lamps showed that they gave an average of 22.8 candle power per lamp. The watts required were 2.07 per candle power. What current did each lamp require? What was the resistance of the lamp?

16. An experimental 100-volt tungsten lamp, tested at the Bureau of Standards, Washington, D.C., required a current of 0.644 ampere and gave 68 candle power. How many watts did it require per candle power? What was the resistance of the lamp?

17. A 1000-volt current of 3 amperes is sent through the primary of a transformer having an efficiency of 97 per cent. How many 104-volt lamps can it light if each requires 0.52 ampere?

18. On the first floor of a house 12, 40-watt, Mazda (tungsten) lamps are used. On the second floor 18, 25-watt, lamps of the same kind are used. How much current will be required to run all the lamps at the same time, if they are run on a 110-volt circuit ?

CHAPTER X

LIGHT

I. NATURE AND INTENSITY OF LIGHT

478. Light is the form of radiant energy which, by its effect upon the retina, excites the sensation of vision. There are many reasons for supposing that this action is the vibration of the ether. This vibration, unlike sound vibration, is transverse, that is, perpendicular to the direction in which the light is moving.

479. Luminous Bodies are those that emit light. The term is usually applied to those bodies only that are selfluminous. When light falls upon a body, part of it is reflected; part is absorbed by the body, changing the molecular or atomic force; and part may or may not pass through the body. Transparent bodies are those that permit light to pass through them in such a way that objects are distinctly visible through them. When light comes through a body as diffused light, and objects cannot be distinctly seen through it, the body is called translucent. If the body does not permit light to pass through at all, it is called an opaque body.

The classification of bodies as opaque and transparent is not very accurate, for thick layers of transparent bodies absorb a great deal of light, while thin layers of opaque substances are sometimes transparent.

Demonstration.

The transparency of gold leaf can readily be shown by laying a piece of gold leaf upon a glass plate and covering it with another plate of the same size. It is well to bind these together with a strip of gummed paper such as is used in making lantern slides. Hold the plates close to the eye and objects can be seen clearly through the gold leaf. Compare the color of the transmitted light with the color that is reflected from gold.

480. The Propagation of Light. A luminous ray is a single line of light propagated from a luminous point. It is always perpendicular to the front of the advancing light wave, and in the case of a spherical wave is a radius drawn from the source of light. A group of rays from the same source is a pencil of rays. If these rays are parallel, they constitute a beam of light. If they diverge from a point, they form a diverging pencil. If they meet at a point, they form a converging pencil.

As long as the medium through which light moves is homogeneous, the direction of propagation is in straight lines. This propagation of light in straight lines is seen when the sun shines through a shutter into a darkened room. In this case the floating dust marks the path of the rays.

481. Umbra and Penumbra.

Demonstration. - Select a

board about 2 ft. long and 8 in. wide, and set up in the middle of

FIG. 446

it a wooden cylinder 2 in. in diameter and 5 in. high. Fasten an upright 6 in. high to one end of the board and near the other end set up two candles 4 in. apart. Observe the shadows cast by the cylinder upon the upright.

The dark part where the shadows overlap is the umbra, and the part which is lighted by only one candle is the penumbra. When a single source of light is so small as to

be considered a point, the shadow is all umbra. If the source of the light is larger, the umbra is partly or completely surrounded by a penumbra. Figure 447 illustrates the case in which the light comes from a luminous ball and the opaque body is a larger ball. If L is the luminous and B the opaque ball, a screen S will show the existence of a circular umbra or shadow whose limits can be determined by moving a straight line around both balls and tangent to both of them

L

B

FIG. 447

on the same side, as LB. There will be a penumbra, however, entirely around the umbra; this will extend to the limits of a circle marked by a line tangent to both balls, but always on opposite sides of them, as CB. At the edge of the umbra the penumbra will be nearly as dark as the umbra, and it will gradually grow lighter and lighter toward the outer edge. The moon is much smaller than the sun, its diameter being 2163 miles, but its distance from the earth is so much less, that it appears to be of nearly the same size. In its journey around the earth the moon sometimes comes between us and the sun and acts as a screen, cutting off the sun's light from us. If the observer is at a part of the earth where the light of the sun is entirely cut off, as in the

umbra of Fig. 447, the eclipse is total. If he is where a portion of the sun can be seen, he is in the penumbra and the eclipse is partial, as in Fig. 448.

482. The Formation of Images. - An image is the picture of an object formed by rays of light coming from it. The image formed by rays passing through a small opening is always inverted. That this must be so is shown by a study of Fig. 449, which illustrates the formation of an image by a

pinhole camera. Since all the light that forms the image comes through the pinhole, all the rays that are not parallel must cross in passing through it. The ray coming from the top of the object forms its image at the bottom. The ray from the bottom of the object goes to the top of the image. The ray from the right side strikes the left, etc. Hence there is a complete reversal in the image.