‘Of Water Drops and Atomic Nuclei: Analogies and Pursuit Worthiness in Science’

New features of the primary rainbow have been revealed in recent experiments by Marston and co-workers in which light is incident on levitated water drops. The drops are oblate spheroids. This paper provides a treatment of the problem by geometrical optics, using the principles of catastrophe optics. It predicts analytically the axial ratios of the drops necessary to produce certain landmark features of the caustics, such as two hyperbolic umbilic foci on the equatorial plane, two kinds of lips events and two E 6 catastrophes. The sequence of caustics produced as the axial ratio of a water drop is changed is shown to be organized by two higher-order singularities which would correspond to drops of refractive index 2 and axial ratios of 1 and √⅔ respectively. The unfoldings of these two higher singularities explain qualitatively all the significant events for a water drop. They also lead to the prediction that two previously unsuspected hyperbolic umbilic foci will be formed in the vertical plane of symmetry when the spheroid is prolate with axial ratio 1.625. Rainbow scattering, that is, the formation of caustics, can occur whenever photons or other particles are scattered by atomic nuclei, atoms, molecules or crystals. Ion channelling through crystals, and the scattering of atoms and molecules by crystal surfaces are examples. The approach described here, of locating higher organizing centres, is equally applicable throughout this broader field.

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Attempts to detect the interaction of neutrons with electrons

Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character ◽

10.1098/rspa.1932.0114 ◽

1932 ◽

Vol 136 (830) ◽

pp. 727-734 ◽

Cited By ~ 18

Keyword(s):

Ion Pairs ◽

Maximum Velocity ◽

Maximum Energy ◽

General Distribution ◽

Expansion Chamber ◽

Atomic Nuclei ◽

Water Drops ◽

Recoil Atoms ◽

The Individual ◽

The Impact

The present paper contains an account of investigations made with a Wilson chamber on the penetrating radiation emitted by beryllium when the latter is bombarded by the α -particles of polonium. Dr. Chadwick has suggested that this radiation consists of a stream of neutrons of unit mass and maximum velocity 3·3 X 10 9 cm. per second. The neutrons in their passage through matter collide occasionally with the atomic nuclei and produce recoil atoms of short range and great ionising power. The recoil atoms of nitrogen have been studied in detail by Dr. Feather, using an automatic expansion chamber, and the lengths of the recoil tracks are in agreement with the neutron hypothesis. It is of special interest to examine the interaction of the neutrons with electrons. The impact of a similar neutron of the same velocity with an electron might be expected to communicate to the latter velocities up to a maximum of 6·6 X 109 cm. per second, that is, twice the velocity of the neutron for a direct collision, corresponding to a maximum energy of the recoil electron of approximately 13,000 electron volts. According to the data of Nuttall and Williams such an electron has a range of recoil of 3·4 mm. As, however, the total ionisation in this length of track would be only 350 ion pairs the valve counter and automatic expansion chamber are unsuitable for its detection. Further, as will be shown later, it is impossible to detect with certainty the occurrence of such recoil electrons produced by neutrons unless the individual water drops produced in the expansion chamber are clearly photographed. For such experiments, and to a still greater degree for the experiments described in 4, it is also necessary that there shall be no appreciable background of drops in the chamber. Even in the absence of ionisation such a general distribution of drops is usually produced unless special precautions are taken. The detection of individual ions by the condensation on them of water vapour and the photography of the individual drops thus produced, while avoiding the presence of water drops which are not associated with ions, has been a special study of the writer working under the supervision of Professor C. T. R. Wilson.

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