Abstract
IN a recent letter Prof. Hartree1 has directed attention to certain difficulties in the theory of refraction as applied to ionised media, and has criticised previous letters by Tonks2 on the subject. I have myself been occupied for some time in trying to clear up this matter, and can confirm Hartree's opinion of the subtlety of the subject, but at the same time it should be said that, in spite of the defective proof, Tonks's result is certainly right. The problem is whether the formula of Sellmeier or that of Lorentz should be applied for a gas composed of free electrons moving among ionised atoms, which may be taken as fixed protons without losing the point of the question. The refractive index n is to be derived from atomic characters, and the problem is whether it is, or which is directly related to these. The alternatives for the ionosphere are whether it is S or L that is equated to where N is the number of electrons per c.c., the frequency of incident waves, and e and m, the charge and mass of the electron respectively. If the formulæ are used to estimate the actual electron density of the ionosphere, there is a discrepancy of 50 per cent according to which of them is adopted; so that the question is by no means trivial. The same problem arises with even greater force in connexion with the optics of metals. It has been discussed by Kronig and Groenewold3; their defence of the use of S is open to exactly the same criticism as that of Tonks, but from the known values of the optical constants of metals it is here even more certain that S is the correct form.
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NATURE, 132, 929, Dec. 16, 1933.
NATURE, 132, 101, July 15, and 710, Nov. 4. See also a letter by Norton, ibid., p. 676, which seems open to the same criticisms, though his method is not very fully described.
Proc. Amst. Akad., 30, 974; 1932.
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DARWIN, C. Refraction of Ionised Media. Nature 133, 62 (1934). https://doi.org/10.1038/133062a0
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DOI: https://doi.org/10.1038/133062a0
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