Modern scientific ideas : especially in the idea of discontinuity / by Sir Oliver Lodge, F.R.S.

  • Lodge, Oliver, Sir, 1851-1940.
Date:
1927
Catalogue details

Licence: In copyright

Credit: Modern scientific ideas : especially in the idea of discontinuity / by Sir Oliver Lodge, F.R.S. Source: Wellcome Collection.

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    must have been atomic absorption; we know that an electron has jumped up from one orbit to another in a particular kind of atom. We can specify the kind of atom and we can specify the kind of jump. If an electron cannot jump, it cannot absorb any radiation; it can only jump by receiving a definite wave-length; but if it does receive that wave-length and does absorb it, it will quench that particular kind of lighf from the sun We shall see a spectrum crossed by dark lines, by reason of the jumps up, and they will fee in exactly the same position as would have been emitted had the jumps been down. So long as we get definite indications of wave¬ length it does not matter whether we get it by a bright line or a dark line; the information given is the same, Whatever an atom can emit, that also it can absorb. We may receive either emission spectra or absorption spectra. One is as good as the other, and both enable the same inferences to be made from the indications of our wave' meter or spectroscope. But it may be said, Are the lines which we observe in the spectrum always accurately in their right position? Is there nothing which can change the wave-length slightly as it travels? The answer is not completely No, for it is possible for the quanta of radiation to encounter free electrons as they travel through space, and thus be made to rebound with a slightly changed wave-length, But then those portions which rebound will not reach us direct, so that for all practical purposes the answer is No. The direct wave arrives unchanged in length, as it was emitted. But suppose it were emitted by a moving body, one which was either approaching or receding; there would be a change then. For it is well known that a railway whistle may be heard about a semitone either above or below its true pitch when a locomotive is either approaching or receding. And that does happen in light also. But it is not a single line that will be tnus shifted: it will be the whole series of lines emitted by a definite kind of atom. And accordingly the effect does not