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.
[Illustration: 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?
[Illustration: 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.