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
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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.
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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.
[Illustration: 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.
{10}
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).
[Illustration: 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).
[Illustration: 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.
[Illustration: FIG. 4]
+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.
[Illustration: FIG. 5.]
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."
[Illustration: FIG. 6.]
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.
[Illustration: FIG. 7.]
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.
{17}
+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.
[Illustration: FIG. 8.]
+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.
[Illustration: FIG. 9.]
+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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