THE MEANING OF THE SPECTRUM
+The Spectrum. Dispersion.+--When a narrow beam of white light is
transmitted through a prism of glass or of any other transparent
substance, it is deflected from its original direction and is at the
same time spread out into a small fan of rays instead of remaining a
single ray. If a screen is placed in the path of these rays a coloured
band is formed on it, the least deflected part of the band being red
and the colours ranging from red through orange, yellow, green, blue,
and indigo, to violet at the most deflected end of the band. This band
of colours is called the spectrum of the white light used, and the
spreading out of the rays is called dispersion.
+Newton's Experiment.+--Newton first discovered this fact with an
arrangement like that in Fig. 17.
[Illustration: FIG. 17.]
If by any means the fan of coloured rays be combined again into a
single beam, white light is reformed, and Newton therefore came to the
conclusion that white light was a mixture of the various colours in the
spectrum, and that the only function of the prism was to separate the
constituents. Of the nature of the constituents Newton had little
knowledge, since he had rejected the wave theory, which could alone
give the clue.
{30}
We now believe that white light is an irregular wave, and that the
prism manufactures from it the Fourier's series of waves to which it is
equivalent. It is supposed that the manufacture is effected by means
of the principle of resonance. As an example of resonance let a small
tap be given to a pendulum just as it commences each swing. Then
because the taps are so timed that each of them increases the swing of
the pendulum by a small amount, they will very soon cause the pendulum
to swing very violently even though the effect of a single tap can
scarcely be detected at all.
Thus when any body which has a free period of vibration is subject to
periodic impulses of the same period as its own, it will vibrate very
vigorously and absorb nearly all the energy of the impulses.
+Electrons and their Vibrations.+--There is conclusive evidence to show
that in the atoms of all substances, and therefore of the glass of
which the prism is composed, there are a number of minute negatively
electrified particles which are called electrons. These are held in
position by a positive charge on the rest of the {31} atom, and if they
are displaced from their usual positions by any means they will vibrate
about these positions. The time of vibration of the electron will
depend upon its position in the atom and upon the position of
neighbouring atoms. In solid or liquid bodies the neighbouring atoms
are so near that they have a considerable influence in modifying the
period of an electron or a system of electrons, and consequently we may
find almost any period of vibration in one or other of these electrons
or systems.
As the wave of light with its alternating electric fields comes up to
the prism, the field will first displace the electrons in one direction
and then in the other, and so on. If the period of one particular type
of electron happens to coincide with the period of the wave, that
electron will vibrate violently and will in its turn send out a series
of waves in the glass. If the wave is an irregular one it will start
all the electrons vibrating, but those electrons will vibrate most
violently whose periods are equal to the periods of the Fourier's
constituents which have the greatest energy. Thus we shall actually
have the Fourier's constituent waves separated into the vibrations of
different electrons. But the speed with which any simple wave travels
in glass or in any transparent medium, other than a vacuum, is
dependent upon its period.
The shorter the period, _i.e._ the shorter the wave-length, the slower
is the speed in most transparent substances. But the slower the speed
in the prism the more is the ray deviated, and therefore we conclude
that the violet end of the spectrum consists of the shortest waves
while the red end consists of the {32} longest waves, and that the
different parts of the spectrum are simple waves of different period.
+The Whole Spectrum.+--The visible spectrum is by no means the whole of
the series of Fourier's waves, however. The eye is sensitive only to a
very small range of period, while there exists in sunlight a range many
times as great.
Those waves of shorter period than the violet end of the visible
spectrum will be deviated even more than the violet, and will therefore
be beyond the violet. They are called the ultra violet rays, and can
easily be detected by means of their chemical activity. They cause a
number of substances to glow, and therefore by coating the screen on
which the spectrum is received with one of these substances, the violet
end of the spectrum is extended by this glow.
The waves of longer period than the red rays will be deviated less than
the red, and will therefore lie beyond the red end of the visible
spectrum. They are called the infra-red rays, and are chiefly
remarkable for their heating effect.
All the rays are absorbed when they fall on to a perfectly dull, black
surface, and their energy is converted into heat. This heating effect
provides the best way of measuring the energy in the different parts of
the spectrum, and of thus constructing curves similar to those given in
Fig. 16. The instrument moat commonly used is called Langley's
bolometer. It consists of a fine strip of blackened platinum, which
can be placed in any part of the spectrum at will and thus absorb the
waves over a very small range of wave-length. It is heated by {33}
them, and the rise in temperature is found by measuring the electrical
resistance of the strip. The electrical resistance of all conductors
varies with the temperature, and since resistance can be measured with
extreme accuracy this forms a very sensitive and accurate method.
+Spectrum of an Incandescent Solid or Liquid.+--The spectra given by
different sources of light show certain marked differences.
An incandescent solid or liquid gives a continuous spectrum, _i.e._ all
the different wave-lengths are represented, but the part of the
spectrum which has the greatest energy is different for different
substances and for different temperatures: cf. arc and gas flame in
Fig. 16. This is quite in keeping with the idea already suggested that
in solids and liquids there are electrons of almost every period of
vibration. When they are agitated by being heated, a mixture of simple
waves of all periods will be sent out giving a very irregular wave.
Gases may also become incandescent. Thus when any compound of sodium
is put into a colourless flame the flame becomes coloured an intense
yellow. This is due to the vapour of sodium, and the agitation of the
electrons in it is probably due to the chemical action in which the
compound is split up into sodium and some other parts.
We may also make the gas incandescent by enclosing it at low pressure
in a vacuum tube and passing an electrical discharge through it. The
glow in the tube gives the spectrum of the gas. Incandescent gases
give a very characteristic kind of spectrum. {34} It consists usually
of a limited number of narrow lines, the rest of the spectrum being
almost perfectly dark. The light therefore consists of a few simple
waves of perfectly definite period. This would suggest that in the
atom of a gas there are only a few electrons which are concerned in the
emission of the light waves.
Thus the spectra of gases and of incandescent solids are represented in
character by the curves in Fig. 18.
[Illustration: FIG. 18.]
+Spectrum Analysis.+--The lines in a gas spectrum are so sharply
defined and are so definitely characteristic of the particular gas that
they serve as a delicate method of detecting the presence of some
elements. These spectra which are emitted by incandescent bodies are
called emission spectra. But not only do different materials emit
different kinds of light when raised to incandescence, but they also
absorb light differently when it passes through them.
When white light is passed through some transparent solids or liquids
and then through a prism, it is found that whole regions of the
spectrum are absent. Thus a potassium permanganate solution {35} which
is not too concentrated absorbs the whole of the middle part of the
spectrum, allowing the red and blue rays to pass through. Since with
solids and liquids the absorbed regions are large and somewhat
ill-defined, the absorption spectra are not of any great use in the
detection of substances.
The absorption spectra of gases show the same sharply defined
characteristics as the emission spectra. Thus if white light from an
arc lamp passes through a flame coloured yellow with sodium vapour, the
spectrum of the issuing light has two sharply defined narrow dark lines
close together in the yellow part of the spectrum in exactly the same
position as the two bright yellow lines which incandescent sodium
vapour itself gives out. The flame has therefore absorbed just those
waves which it gives out. This is perfectly general, and applies to
solids and liquids as well as to gases. It is perfectly in keeping
with our view of the refraction of light by the resonance of electrons
to the Fourier's constituents which have the same period. For if the
electrons have a certain period of vibration they will resound to waves
of that period and therefore absorb their energy.
+Spectrum of the Sun.+--One of the most interesting examples of the
absorption by incandescent gases of their own characteristic lines is
provided by the sun. The spectrum of the sun is crossed by a large
number of fine dark lines which were mapped out by Fraunhöfer and are
therefore called Fraunhöfer lines. These lines are found to be in the
position of the characteristic lines of a number of known elements,
{36} and therefore we assume that these elements are present in the
sun. The interior of the sun is liquid or solid owing to the pressure
of the mass round it. It therefore emits a continuous spectrum. But
the light has to pass through the outer layers of incandescent vapour,
and these layers absorb from the light their characteristic waves and
so produce the dark lines in the spectrum.
The spectra of stars show similar characters to those of the sun, and
therefore we assume them to be in the same condition as the sun.
The spectra of nebulæ consist only of bright lines, and we therefore
assume that nebulæ consist of incandescent masses of gas which have not
yet cooled enough to have liquid or solid nuclei.
{37}