The smooth surfaces of houses often reflect so much of the light falling upon them that the glare is thrown into the windows of surrounding houses into which the sun itself cannot shine. If one stands in the right position, the reflection of trees and other objects can be seen in a smooth lake. But the reflection cannot be seen if the position of the spectator is much changed. The reflected ray must therefore maintain a certain relation to the ray that strikes the surface from the object.
In Experiment 33, when the pencil was held perpendicular to the mirror at the point where the rays touched the mirror, it was seen that both the ray from the window and the reflected ray made about the same angle with it. These two angles are respectively called the angle of incidence and the angle of reflection. By most careful experimentationFig. 37.
it has been found that the angles between each of these two rays, and the line drawn perpendicularly to the reflecting surface are always equal, or in other words the angleof reflection is always equal to the angle of incidence. This explainswhy, if you are standing in a room beyond one side of a mirror, you can see in the mirror only the opposite side of the room.
33.The Speed of Light. -In the latter part of the seventeenthcentury a Danish astronomer by the name of R?mer, after carefully watching the brightest of Jupiter"s satellites or moons as it revolved around the planet, noticed that the timeof occurrence of its eclipses or passages behind the planet showed a peculiar variation. He accurately determined the interval between two eclipses or thetime it took for a complete revolution of the satellite around the planet.
Fig. 38.
Using this interval he computed the time at which other eclipses should take place and found that as the earth in its revolution around the sun moved away from Jupiter the eclipses appeared to take place more and more behind time. Determining the exact time at which an eclipse took place when the earth was nearest to Jupiter, and computing the time an eclipse should take place six months later when the earth was farthest from Jupiter, he found that the actual time of the eclipse was 22 minutes behind the computed time. This slowness he said must be due to the time required by the light in crossing the earth"s orbit.
Many determinations of this kind have been made since those of R?mer, and it has been found that he was somewhat in error, as the time required by light in traveling across the earth"s orbit is about 16 minutes and 40 seconds, or 1000 seconds. Since the diameter of the earth"s orbit is about 186,000,000 miles the speed of light must beabout 186,000 miles per second. Determinations of the speed of light have been made in several other ways with almost like results.
34.Theories Concerning Light. -Although it is very easy to per-ceive light and to examine many of its properties, yet to determine just what it is that produces the light sensation has been found vastly dif- ficult. Sir Isaac Newton thought that light consisted of streams of very minute particles, or corpuscles, thrown off by the luminous body. Since about 1800, it has been considered a form of wave motion which is transmitted through the ether which fills all space.
35.Sound.
Experiment 34. -Arrange a large widemouthed bottle with a small bell suspended in it from the stopper and a delivery tube extending through the stopper. Attach the delivery tube by a thick-walled rubber tube to an air pump and exhaust the air from the bottle. Shake the bottle so that the bell can be seen to ring but does not strike the sides of the bottle. Can the sound be heard distinctly?
Fig. 39.
Although sound is not related to the sun"s energy it seems best for certain reasons to consider it briefly in this place. In Experiment 34, it was found that if the air was exhausted and the bell did not touch the sides of the bottle, almost no sound was heard when the dapper of the bell showed that the bell was ringing. This shows that the sounds we usually hear are transmitted in some way by the aid of the air.
Sound is found to be a vibratory wave motion in a material medium. If a scratch is made on the end of a long log, it can be heard if the ear is placed at the other end of the log, when it cannot be heard if the ear is away from the log. In this case the medium is the wood.
If a stone is dropped into a quiet pond, the rippling waves developed will extend often to theFig. 40.
farthest shore of the pond, but a chip floating nearwhere the stone fell will not be moved from its position except up and down. Thus the waves traveled outward from the point of origin, but there was no outward movement of the water. If a long rope, attached at one end and held in a horizontal position, is suddenly struck with a stick, a wave motion will travel along the rope from end to end, but the particles of the rope will simply move up and down. It is in a similar way to this that the sound waves travel, but the particles which transmit the sound only move back and forth through small distances.
Summary. -All energy upon the earth is due to the sun. There are two kinds of energy, kinetic and potential. Energy may be changed in countless ways but it cannot be destroyed.
Heat is a form of molecular energy. Heat is shown in changes intemperature and these are measured by thermometers, of which the Fahrenheit and the Centigrade are the commonest. Heat affects the state of matter: the same substance may be solid, liquid or gaseous, depending on the amount of heat to which it has been subjected. This is shown in ice, water and steam. Heat is transferred by conduction, convection and radiation. The unit for measuring the amount of heat is called the calorie.