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.

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.

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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).

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 {11} 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).

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 becomes inappreciable again. The effect is the same as a perfectly elastic ball bouncing on a perfectly smooth surface, and consequently the angle to the surface at which the corpuscle comes up is equal to the angle at which it departs.

+Refraction.+--To explain refraction, it is supposed that when the corpuscle comes very close to the surface of the transparent substance it is attracted by the denser substance, e.g. glass, more than by the lighter substance, e.g. air. Thus a particle moving along the dotted line in air (Fig. 4) would reach the {12} point A before the attraction becomes appreciable, and therefore would be moving in a straight line. Between A and B the attraction of the glass will be felt and will therefore pull the particle round in the path indicated. Beyond B, the attraction again becomes inappreciable, because the glass will attract the particle equally in all directions, and therefore the path will again become a straight line. We notice that by this process the direction of the path has become more nearly normal to the surface, and this is as it should be. Further, by treating the angles between the two paths and the normal mathematically we may deduce the laws of refraction which have been obtained experimentally. One other important point should be noticed. Since the surface has been attracting the particle between A and B the speed of the particle will be greater in the glass than in the air.

+Ejection and Refraction at the same Surface.+--A difficulty very soon arises from the fact that at nearly all transparent surfaces some light is reflected and some refracted. How can the same surface sometimes repel and sometimes attract a corpuscle? Newton surmounted this difficulty by attributing a polarity to each particle, so that one end was repelled and the other attracted by the reflecting and refracting {13} surface. Thus, whether a particle was reflected or refracted depended simply upon which end happened to be foremost at the time. By attributing suitable characteristics to the corpuscles, Newton with his superhuman ingenuity was able to account for all the known facts, and as the corpuscles were so small that direct observation was impossible, and as Newton's authority was so great, there was no one to say him nay.

+Wave Theory. Rectilinear Propagation.+--True, Huyghens in 1678 had propounded the theory that light consists of waves of some sort starting out from the luminous body, and he had shown how readily it expressed a number of the observed facts; but light travels in straight lines, or appears to do so, and waves bend round corners and no one at that time was able to explain the discrepancy. Thus for nearly a century the theory which was to be universally accepted remained lifeless and discredited. The answer of the wave theory to the objection now is, that light does bend round corners though only slightly and that the smallness of the bend is quite simply due to the extreme shortness of the light waves. The longer waves are, the more they bend round corners. This can be noticed in any harbour with a tortuous entrance, for the small choppy waves are practically all cut off whereas a considerable amount of the long swell manages to get into the harbour.

+Interference of Light. Illustration by Ripples+.--The revival of the wave theory dates from the discovery by Dr. Young of the phenomenon of interference of light. In order to understand this we will {14} consider the same effect in the ripples on the surface of mercury. A tuning-fork, T (Fig. 5), has two small styles, S S, placed a little distance apart and dipping into the mercury contained in a large shallow trough. When the tuning-fork is set into vibration, the two styles will move up and down in the mercury at exactly the same time and each will start a system of ripples exactly similar to the other. At any instant each system will be a series of concentric circles with its centre at the style, and the crests of the ripples will be at equal distance from each other with the troughs half-way between the crests.

The ripples from one style will cross those from the other, and a curious pattern, something like that in Fig. 6, will be formed on the mercury. S S represents the position of the two styles, while the plain circles denote the positions of the crests and the dotted circles the positions of the troughs at any instant. Where two plain circles cross it is evident that both systems of ripples are producing a crest, and so the two produce an exaggerated crest. Similarly where two dotted circles cross an exaggerated trough is produced. Thus in the shaded portions of the diagram we get more violent ripples than those due to a single style. Where a plain circle cuts a dotted one, however, one system of ripples produces a {15} crest and the other a trough, and between them the mercury is neither depressed below nor raised above its normal level. At these points, therefore, the effect of one series of ripples is just neutralised by the effect of the other and no ripples are produced at all. This occurs in the unshaded regions of the diagram.

The mutual destruction of the effects of the two sets of waves is "Interference."

Now imagine a row of little floats placed along the line EDCBABCDE. At the lettered points the floats will be violently agitated, but at the points midway between the letters they will be unmoved. This exactly represents the effect of two interfering sources of light S, S, sending light which is received by a screen at the dotted line EDCBABCDE. The lettered points will be brightly illuminated while the intermediate points will be dark.

In practice it is found impossible to make two {16} sources of light whose vibrations start at exactly the same time and are exactly similar, but this difficulty is surmounted by using one source of light and splitting the waves from it into two portions which interfere.

+Young's Experiment.+--Dr. Young's arrangement is diagrammatically represented in Fig. 7.

Light of a certain wave length is admitted at a narrow slit S, and is intercepted by a screen in which there are two narrow slits A and B parallel to the first one.

A screen receives the light emerging from the two slits. If the old corpuscular theory were true there would be two bright bands of light, the one at P and the other at Q, but instead Dr. Young observed a whole series of parallel bright bands with dark spaces in between them. Evidently the two small fractions of the original waves which pass through A and B spread out from A and B and interfere just as if they were independent sources like the two styles in the mercury ripples experiment.

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+Speed of Light in Rare and Dense Media.+--The discovery of interference again brought the wave theory into prominence, and in 1850 the death-blow was given to the corpuscular theory by Foucault, who showed that light travels more slowly in a dense medium such as glass or water than in a light medium such as air. This is what the wave theory anticipates, while the reverse is anticipated by the corpuscular theory.

But if light and heat consist of waves, what kind of waves are they and how are they produced?

+Elastic Solid Theory.+--In the earlier days of the wave theory it was supposed that the whole of space was filled with something which acted like an elastic solid material in which the vibrations of the atoms of a luminous body started waves in all directions, just as the vibrations of a marble embedded in a jelly would send out waves through the jelly. These waves are quite easily imagined in the following way.

If one end of an elastic string be made to oscillate to and fro a series of waves travels along the string. If a large number of these strings are attached to an oscillating point and stretch out in all directions, the waves will travel along each string, and if the strings are all exactly alike will travel at the same speed along all of them. Any particular crest of a wave will thus at any instant lie on the surface of a sphere whose centre is the oscillating point. If now we imagine that the strings are so numerous that they fill the whole of the space we have a conception of the transmission of waves by an elastic solid.

+Electromagnetic Waves.+--Since Maxwell published {18} his electromagnetic theory in 1873 it has been universally held that heat and light consist of electro-magnetic waves.

These are by no means so easy to imagine as the elastic waves, as there is no actual movement of the medium; an alternating condition of the medium is carried onward, not an oscillation of position.

When a stick of sealing-wax or ebonite is rubbed with flannel it becomes possessed of certain properties which it did not have before. It will attract light pieces of paper or pith that are brought near to it, it will repel a similar rubbed piece of sealing-wax or ebonite and will attract a rod of quartz which has been rubbed with silk.

The quartz rod which has been rubbed with silk has the same property of attracting light bodies which the ebonite and sealing-wax rod has, but it repels another rubbed quartz rod and attracts a rubbed ebonite or sealing-wax rod.

+Positive and Negative Electrification.+--The ebonite is said to be negatively electrified and the quartz positively electrified.

When the two rods, one positively and the other negatively electrified, are placed near to one another, we may imagine the attraction to be due to their being joined by stretched strings filling up all the space around them. If a very small positively electrified body be placed between the two it will tend to move from the quartz to the ebonite, _i.e._ in the direction of the arrows.

+The Electric Field. Lines of Force.+--The space {19} surrounding the electrified sticks in which the forces due to them are appreciable is called the electric field, and the direction in which a small positively electrified particle tends to move is called the direction of the field. The lines along which the small positive charge would move are called lines of force.

The conception of the electric field as made up of stretched elastic strings is, of course, a very crude one, but there is evidently some change in the medium in the electric field which is somewhat analogous to it.

+Electric Oscillations.+--If the position of the two rods is reversed, then of course the direction of the field at a point between them is reversed, and if this reversal is repeated rapidly, we shall have the direction of the field alternating rapidly. If these alternations become sufficiently rapid they are conveyed outwards in much the same way as the oscillations of position are conveyed in an ordinary ripple. Thus suppose the two rods are suddenly placed in the position in the diagram. The field is not established instantaneously, the lines of force taking a short time to establish themselves in their ultimate positions. During this time the lines of force will be travelling outwards to A in the direction of the dotted arrow. {20} Before they reach A let us suppose that the position of the rods is reversed. Then the direction of the lines is reversed and these reversed lines will travel outwards towards A, following in the track of the original lines. Thus a continuous procession of lines of force, first in one direction and then in the opposite direction, will be moving out perpendicular to themselves in the direction of the dotted arrow.

This constitutes an electric wave.

+Magnetic Oscillation, Lines of Force, and Field+.--Almost exactly the same kind of description applies to a magnetic wave. The space near to the North and South poles of a magnet is modified in somewhat the same way as that between the electrified rods, and the magnetic lines of force are the lines along which a small North magnetic pole would move. We may imagine a rapid alternation of the magnetic field by the rapid reversal of the positions of the North and South poles, and we may imagine the transmission of the alternations by means of the procession of magnetic lines of force.

+Changes in Magnetic Field.+--But experiment shows that whenever the magnetic field at any place is changing an electric field is produced during the alteration, and _vice-versa_. Electric and magnetic waves must therefore always accompany one another, and the two sets of waves together constitute electro-magnetic waves.

These are the waves which a huge amount of experimental evidence leads us to believe constitute heat, light, the electric waves used in wireless telegraphy, {21} and the invisible ultraviolet waves which are so active in inducing chemical action.

+Oscillation of Electric Charges within the Atom.+--We have seen how these waves might be produced by the oscillation of two electrified rods, and it is supposed that the light coming from luminous bodies is produced in a similar way. There are many reasons for believing that there exist in the atoms of all substances, minute negatively electrified particles which may rotate in small orbits or oscillate to and fro within the atom. There also exists an equal positive charge within the atom. As the negative particles rotate or oscillate in the atom, it is evident that the field between them and the positively electrified part of the atom alternates, and so electro-magnetic waves are sent out.

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