HEAT AND LIGHT AS COMPANIONS
"The night has a thousand eyes,
And the day but one;
Yet the light of the bright world dies
With the dying sun."
136. Most bodies which glow and give out light are hot; the stove which glows with a warm red is hot and fiery; smoldering wood is black and lifeless; glowing coals are far hotter than black ones. The stained-glass window softens and mellows the bright light of the sun, but it also shuts out some of the warmth of the sun's rays; the shady side of the street spares our eyes the intense glare of the sun, but may chill us by the absence of heat. Our illumination, whether it be oil lamp or gas jet or electric light, carries with it heat; indeed, so much heat that we refrain from making a light on a warm summer's night because of the heat which it unavoidably furnishes.
137. Red a Warm Color. We have seen that heat and light usually go hand in hand. In summer we lower the shades and close the blinds in order to keep the house cool, because the exclusion of light means the exclusion of some heat; in winter we open the blinds and raise the shades in order that the sun may stream into the room and flood it with light and warmth. The heat of the sun and the light of the sun seem boon companions.
We can show that when light passes through a prism and is refracted, forming a spectrum, as in Section 127, it is accompanied by heat. If we hold a sensitive thermometer in the violet end of the spectrum so that the violet rays fall upon the bulb, the reading of the mercury will be practically the same as when the thermometer is held in any dark part of the room; if, however, the thermometer is slowly moved toward the red end of the spectrum, a change occurs and the mercury rises slowly but steadily, showing that heat rays are present at the red end of the spectrum. This agrees with the popular notion, formed independently of science, which calls the reds the warm colors. Every one of us associates red with warmth; in the summer red is rarely worn, it looks hot; but in winter red is one of the most pleasing colors because of the sense of warmth and cheer it brings.
All light rays are accompanied by a small amount of heat, but the red rays carry the most.
What seems perhaps the most unexpected thing, is that the temperature, as indicated by a sensitive thermometer, continues to rise if the thermometer is moved just beyond the red light of the spectrum. There actually seems to be more heat beyond the red than in the red, but if the thermometer is moved too far away, the temperature again falls. Later we shall see what this means.
138. The Energy of the Sun. It is difficult to tell how much of the energy of the sun is light and how much is heat, but it is easy to determine the combined effect of heat and light.
FIG. 89.—The energy of the sun can be measured in heat units.
Suppose we allow the sun's rays to fall perpendicularly upon a metal cylinder coated with lampblack and filled with a known quantity of water (Fig. 89); at the expiration of a few hours the temperature of the water will be considerably higher. Lampblack is a good absorber of heat, and it is used as a coating in order that all the light rays which fall upon the cylinder may be absorbed and none lost by reflection.
Light and heat rays fall upon the lampblack, pass through the cylinder, and heat the water. We know that the red light rays have the largest share toward heating the water, because if the cylinder is surrounded by blue glass which absorbs the red rays and prevents their passage into the water, the temperature of the water begins to fall. That the other light rays have a small share would have been clear from the preceding Section.
All the energy of the sunshine which falls upon the cylinder, both as heat and as light, is absorbed in the form of heat, and the total amount of this energy can be calculated from the increase in the temperature of the water. The energy which heated the water would have passed onward to the surface of the earth if its path had not been obstructed by the cylinder of water; and we can be sure that the energy which entered the water and changed its temperature would ordinarily have heated an equal area of the earth's surface; and from this, we can calculate the energy falling upon the entire surface of the earth during any one day.
Computations based upon this experiment show that the earth receives daily from the sun the equivalent of 341,000,000,000 horse power—an amount inconceivable to the human mind.
Professor Young gives a striking picture of what this energy of the sun could do. A solid column of ice 93,000,000 miles long and 2-1/4 miles in diameter could be melted in a single second if the sun could concentrate its entire power on the ice.
While the amount of energy received daily from the sun by the earth is actually enormous, it is small in comparison with the whole amount given out by the sun to the numerous heavenly bodies which make up the universe. In fact, of the entire outflow of heat and light, the earth receives only one part in two thousand million, and this is a very small portion indeed.
139. How Light and Heat Travel from the Sun to Us. Astronomers tell us that the sun—the chief source of heat and light—is 93,000,000 miles away from us; that is, so far distant that the fastest express train would require about 176 years to reach the sun. How do heat and light travel through this vast abyss of space?
FIG. 90.—Waves formed by a pebble.
A quiet pool and a pebble will help to make it clear to us. If we throw a pebble into a quiet pool (Fig. 90), waves or ripples form and spread out in all directions, gradually dying out as they become more and more distant from the pebble. It is a strange fact that while we see the ripple moving farther and farther away, the particles of water are themselves not moving outward and away, but are merely bobbing up and down, or are vibrating. If you wish to be sure of this, throw the pebble near a spot where a chip lies quiet on the smooth pond. After the waves form, the chip rides up and down with the water, but does not move outward; if the water itself were moving outward, it would carry the chip with it, but the water has no forward motion, and hence the chip assumes the only motion possessed by the water, that is, an up-and-down motion. Perhaps a more simple illustration is the appearance of a wheat field or a lawn on a windy day; the wind sweeps over the grass, producing in the grass a wave like the water waves of the ocean, but the blades of grass do not move from their accustomed place in the ground, held fast as they are by their roots.
If a pebble is thrown into a quiet pool, it creates ripples or waves which spread outward in all directions, but which soon die out, leaving the pool again placid and undisturbed. If now we could quickly withdraw the pebble from the pool, the water would again be disturbed and waves would form. If the pebble were attached to a string so that it could be dropped into the water and withdrawn at regular intervals, the waves would never have a chance to disappear, because there would always be a regularly timed definite disturbance of the water. Learned men tell us that all hot bodies and all luminous bodies are composed of tiny particles, called molecules, which move unceasingly back and forth with great speed. In Section 95 we saw that the molecules of all substances move unceasingly; their speed, however, is not so great, nor are their motions so regularly timed as are those of the heat-giving and the light-giving particles. As the particles of the hot and luminous bodies vibrate with great speed and force they violently disturb the medium around them, and produce a series of waves similar to those produced in the water by the pebble. If, however, a pebble is thrown into the water very gently, the disturbance is slight, sometimes too slight to throw the water into waves; in the same way objects whose molecules are in a state of gentle motion do not produce light.
The particles of heat-giving and light-giving bodies are in a state of rapid vibration, and thereby disturb the surrounding medium, which transmits or conveys the disturbance to the earth or to other objects by a train of waves. When these waves reach their destination, the sensation of light or heat is produced.
We see the water waves, but we can never see with the eye the heat and light waves which roll in to us from that far-distant source, the sun. We can be sure of them only through their effect on our bodies, and by the visible work they do.
140. How Heat and Light Differ. If heat and light are alike due to the regular, rapid motion of the particles of a body, and are similarly conveyed by waves, how is it, then, that heat and light are apparently so different?
Light and heat differ as much as the short, choppy waves of the ocean and the slow, long swell of the ocean, but not more so. The sailor handles his boat in one way in a choppy sea and in a different way in a rolling sea, for he knows that these two kinds of waves act dissimilarly. The long, slow swell of the ocean would correspond with the longer, slower waves which travel out from the sun, and which on reaching us are interpreted as heat. The shorter, more frequent waves of the ocean would typify the short, rapid waves which leave the sun, and which on reaching us are interpreted as light.