The most important part of the study of Radiation of energy is the investigation of the characters of the waves which constitute heat and light, but there is another method of transference of energy included in the term Radiation; the source of the energy behaves like a battery of guns pointing in all directions and pouring out a continuous hail of bullets, which strike against obstacles and so give up the energy due to their motion. This method is relatively unimportant, and is usually treated of separately when considering the subject of Radioactivity. We shall therefore not consider it in this book.
RADIATION
CHAPTER I
THE NATURE OF RADIANT HEAT AND LIGHT
Similarity of Heat and Light.—That light and heat have essentially the same characters is very soon made evident. Both light and heat travel to us from the sun across the ninety odd millions of miles of space unoccupied by any material.
Figure 1
Both are reflected in the same way from reflecting surfaces. Thus if two parabolic mirrors be placed facing each other as in the diagram (Fig. 1), with a source of light L at the focus of one of them, an inverted image of the light will be formed at the focus I of the other one, and may be received on a small screen placed there. The paths of two of the rays are shown by the dotted lines. If L be now replaced by a heated ball and a[[1]] blackened thermometer bulb be placed at I, the thermometer will indicate a sharp rise of temperature, showing that the rays of heat are focussed there as well as the rays of light.