Experiment 30. -Fill a battery jar about two thirds full of water. Place a glass rodFIG. 33.
or stick in the jar. Does the rod appear straight? Pour two or three inches ofkerosene on the top of the water. What effect does this have on the appearance of the rod?
Experiment 31. -Hold an ordinary spectacle lens such as is used by an elderly person, or any convex lens, between the sun and a piece of paper. Vary the distances of the lens from the paper. The heat and light rays from the sun are bent so that they converge to a point. Try the same experiment with a lens used by a short-sighed person, or a concave lens. This lens does not have the same effect as the convex lens. The rays are made to diverge. Why cannot long-sighted and short-sighted persons use the same glasses?
In the experiment of the penny in the dish, the water in some way bent the ray of light and made the penny come into the line of sight when it could not be seen before the water was there. This experiment shows that when light is passing from one medium to another it does not always travel in the same straight line. Certain media offer more resistance to the passage of light than others and are called denser media. It is this resistance which causes the bending of the ray.
Suppose that a column of soldiers marching in company front are passing through a corn field and come obliquely upon a smooth open field. The men as they come on to the open field are unincumbered by the cornstalks and will move faster, and thus the line of march willFig. 34.
swing in toward the edge of the corn field. It can easily be seen that the bending of the line would be in the opposite direction if the soldiers were marching from the smooth field into the corn field. If the company front was parallel to the edge of thecorn field, then the men would reach the open field at the same timeand there would be no swinging of the line.
The above illustration roughly explains what happens when light passes from one medium to another. Refraction is the name given to this bending of light in passing through different media or through a medium of changing density. Twilight, mirage, the flattening of the sun"s disk at the horizon and other appearances we shall find later are due to this property of light.
31.The Intensity of Light.
Experiment 32. -Take two square pieces of paraffin about an inch thick, or better two squares of parawax, and place back to back with a piece of cardboard or tinfoil between them. When a light is placed on either side of this apparatus the wax toward the light willbe illuminated, but not that on the other
side of the cardboard. If lights are placed on each side, it is easy to see when both pieces of wax are equally illuminated, orreceive the same amount of light. In this way the strengths of lights can be compared.
Fig. 35.
Place a candle about 25 cm. in front of one side of this apparatus, and 4 candles, placed close together on a piece of cardboard so that they can be readily moved, about 90 cm. away on the other side. Move these candles back and forth till a position is found where both pieces of wax are illuminated alike. Measure the distance of the four candles from the wax. How many times as far away are they than the one candle?
The brightness of the sun"s light is so great that even an arc light placed in direct sunlight appears like a dark spot. So great, however, is the sun"s distance that the earth receives only a minute portion, less than one two-billionth of the light and heat it gives out. It is impossible to express the greatness of this light in ordinary terms. The standard measure for intensities of light is the candle power. This is the light given out by a standard candle, which is practically our ordinary No. 12 paraffin candle. The ordinary incandescent electric light is sixteen candle power.
No comprehensible figures will express the intensity of the sun, using the candle power as a measure. The intensity of light, like thatFig. 36.
of heat and electricity, and all forms of energy which spread out uniformly from their point of origin, varies inversely as the square of the distance from the source. This rapid decrease in the brightness of light as the distance increases is the reason why so small a change in the distance of a lamp makes so great a difference in the ease with which we can read a book. If we make the distance to the lamp half as great, we increase the amount of light on the book four times (Fig. 36).
32.Reflection of Heat and Light.
Experiment 33. -In a darkened room reflect by means of a mirror, a ray of light from a small hole in the curtain, or from some artificial source of light, on to a plane mirror lying flat upon a table. If there is not sufficient dust in the air to make the paths of the rays apparent, strike two blackboard erasers together near the mirror. Hold a pencil vertical to the mirror at the point wherethe rays strike it. Compare with each other the angle formed by each ray with the pencil. Raise the edge of the mirror, and notice the effect on the reflected ray. Place the pencil at right angles to this new position of the mirror, and compare the angles in each case. How do the sizes of the angles on either side of the pencil compare?
It has already been stated that the moon shines by reflected light. It is a matter of common observation that objects on the earth reflect both heat and light. In the summer, the walls of the houses and the pavements of the streets sometimes reflect the heat to such an extent that it becomes almost unbearable. In countries where the sun shines brightly nearly all of the time, as in the Desert of Sahara, reflectors have been so arranged as to reflect the heat of the sun on to boilers and to run steam engines.
A REFLECTION ENGINE.
This engine used the rays of the sun instead of coal to heat its boiler.