63.Effect of Atmospheric Conditions on Light.
Experiment 75. -Allow sunlight to pass through a glass prism and fall upon a white wall or piece of paper. How has the white sunlight been affected? Where did the colors come from? In what order are the colors arranged? This group of colors into which a prism separates white light is called the spectrum.
If the light that comes from the sun is passed through a glass prism, as in Experiment 75, it will be seen to be composed of many different colors. In fact it is the absorption of some of these colors and the reflecting of others which make objects appear of different colors.
Light itself is a vibration which has the power of affecting the optic nerve, and the different colors are vibrations of different lengths. Now the sunlight is affected by the air through which it comes. If there is smoke or dust in the air, the sun will appear to be red. When the sun sets at night and the rays come to us through a considerableFig. 62.
thickness of air which is near the surface of the earth and contains dust, the light often appears red. On the top of a high mountain or on a clear day or when the sun is high overhead the sky appears blue. Both these colors are due to the effect of the atmosphere on transmitted light.
Sometimes after a shower an arch appears in the heavens composed of beautiful colors; we call this a rainbow. In this case the sunlight isLICK OBSERVATORY.
As light is affected by the atmosphere, observatories must be placed where atmospheric conditions are the best.
This famous observatory is on a mountain in the clear air of California.
broken into different colors by the drops of water which still fall in the distance, just as it is when passing through a prism.
Sometimes the sun or moon is surrounded by bright rings called- when of small diameter-coronas, and when of great diameter, halos. These rings are due to the effect of water or ice particles on the light coming from the sun or the moon.
Under certain conditions it may happen that light coming from objects at a distance is so refracted and reflected by the layers of air of different density, through which it comes to the eye of the observer, that objects appear to be where they are not, like the image of a person seen in a mirror. This phenomenon is called mirage or looming. It occurs most frequently on deserts and over the sea near the coast.
Sometimes in high latitudes arches and streamers of colored light are seen illuminating the northern sky. The brilliancy and colors of the illumination vary. Sometimes it is bright enough to be seen even in the daytime. This display is called the aurora borealis or "northern lights" and is believed to be an electrical phenomenon in thin air. The heights of the streamers have been calculated to be more than a hundred, perhaps several hundred miles, so that it is probable that air in a rare condition extends to this elevation.
64.The Warming of the Atmosphere. -The sun transmits both light and heat to the surface of the earth through the atmosphere. On the top of a high mountain the temperature is found to be colder than on the lower levels. The amount of sun radiation, technically called insolation, that falls upon a given surface on the mountain is about the same as that which falls upon an equal surface in the valley. If the heating effect is less, it must be due to something besides the number of heat rays intercepted.
In the spring when gardeners wish to hurry the growth of their plants, they cover them withboxes, the tops of which are made
Fig. 63.
of glass (Fig. 63). It is found that the temperature within the boxes is higher than that outside. The heat rays coming from the sun are in some way affected by the absorption of the ground so that they are not able to get out through the glass as readily as they get in.
Now the atmosphere affects the heat rays radiating from the earth in the same way that the glass does, and keeps them from flying back into space and leaving the surface cold. Where the atmosphere is thin, as on the mountains, this effect is not so great, and therefore the surface is colder and often covered with snow. When there are clouds in the air, they help to hold in the heat. That is why in the fall, when it is getting cold enough for frosts, the farmers say that the frosts are likely to come on clear nights but not on cloudy ones.
For the same reason plants are covered by pieces of paper and smoky fires are built around cranberry bogs to cover them with an artificial cloud of smoke on nights when there is likely to be a frost. Thus the atmosphere acts as a blanket to the earth and keeps in the heat of the sun just as blankets on a bed keep in the heat of the body. If there were no atmosphere on the earth, its surface would become intensely hot during the day, when the sun shines directly upon it, and intensely cold at night, so that it would not be possible for life to exist.
It has been estimated that, if there were no atmosphere, the meantemperature of the earth"s surface during the day would be 350° F. and during the night -123° F. Thus the atmosphere is not only needed for the breathing of plants and animals and for carrying moisture, but also for keeping in the heat of the sun. On the moon, where there is no atmosphere, there can be no life as we know it.
65.Cause of the Variation in Atmospheric Temperatures.
Experiment 76. -Cut a hole 4 in. square in the center of a board 12 in. square. Fit tightly into this hole one end of a wooden tube 4 in. square and 1 ft. long. Paint the inside and outside ofthe tube a dull black. Hinge the opposite
end of this tube 10 in. from the end of a baseboard 2 ft. long and 16 in. wide, having 6 in. of the board on either side ofthe tube.