RADIATION

BY P. PHILLIPS

D.Sc. (B'HAM), B.Sc. (LONDON), B.A. (CANTAB.)

LONDON: T. C. & E. C. JACK
67 LONG ACRE, W.C., AND EDINBURGH
NEW YORK: DODGE PUBLISHING CO.
1912

CONTENTS

CHAP.

INTRODUCTION

I. THE NATURE OF RADIANT HEAT AND LIGHT

II. GRAPHIC REPRESENTATION OF WAVES

III. THE MEANING OF THE SPECTRUM

IV. THE LAWS OF RADIATION

V. FULL RADIATION

VI. THE TRANSFORMATION OF ABSORBED RADIATION

VII. PRESSURE OF RADIATION

VIII. THE RELATION BETWEEN RADIANT HEAT AND ELECTRIC WAVES

INDEX

INTRODUCTION

We are so familiar with the restlessness of the sea, and with the havoc which it works on our shipping and our coasts, that we need no demonstration to convince us that waves can carry energy from one place to another. Few of us, however, realise that the energy in the sea is as nothing compared with that in the space around us, yet such is the conclusion to which we are led by an enormous amount of experimental evidence. The sea waves are only near the surface and the effect of the wildest storm penetrates but a few yards below the surface, while the waves which carry light and heat to us from the sun fill the whole space about us and bring to the earth a continuous stream of energy year in year out equal to more than 300 million million horsepower.

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.

[1] See page 37.

Both heat and light behave in the same way in passing from one transparent substance to another, e.g. from air into glass. This can be readily shown by forming images of sources of heat and of light by means of a convex lens, as in the diagram (Fig. 2).

FIG. 2.

The source of light is represented as an electric light bulb, and two of the rays going to form the image of the point of the bulb are represented by the dotted lines. The image is also dotted and can be received on a screen placed in that position.

If now the electric light bulb be replaced by a heated ball or some other source of heat, we find by using a blackened thermometer bulb again that the rays of heat are brought to a focus at almost the same position as the rays of light.

The points of similarity between radiant heat and light might be multiplied indefinitely, but as a number of them will appear in the course of the book these few fundamental ones will suffice at this point.

The Corpuscular Theory.—A little over a century ago everyone believed light to consist of almost inconceivably small particles or corpuscles shooting out at enormous speed from every luminous surface and causing the sensation of sight when impinging on the retina. This was the corpuscular theory. It readily explains why light travels in straight lines in a homogeneous medium, and it can be made to explain reflection and refraction.

Reflection.—To explain reflection, it is supposed that the reflector repels the particles as they approach it, and so the path of one particle would be like that indicated by the dotted line in the diagram (Fig. 3).

FIG. 3.

Until reaching the point A we suppose that the particle does not feel appreciably the repulsion of the surface. After A the repulsion bends the path of the particle round until B is reached, and after B the repulsion