CHAPTER III

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.

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.

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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.

+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.

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