Abstract

Brocken bows are sometimes observed in natural clouds as spectrum colored rings surrounding the shadow of one’s head. This phenomenon was reproduced in the laboratory by illuminating an artificial cloud produced by adiabatic expansion in a large cloud chamber. The light scattered in the backward direction by a cloud was observed by transverse reflection in a convex mirror kept in the path of a beam of light incident on the cloud. A reduced image of the shadow of the mirror on the cloud was seen surrounded by a series of spectrum colored rings. About three to four orders of the rings of a given color were often observed. The size of the rings of a given color depends on the size of the water drops in the cloud. The size of the water drops in the cloud was determined by measuring the apertures of the diffraction rings (coronas) seen by transmitted light. It is found that in the case of Brocken rings α sinγ_n=π(n−1.22), where α=2πr/λ, r being the radius of water drops, λ the wavelength of light, and γ_n the angular aperture of the nth-order bright ring.

It is also shown that the size of drops in natural cloud in which Brocken bows are seen, should be near about 13.5 μ.

© 1955 Optical Society of America

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