118. Interference Phenomena in Raindrops
Many people obliged to wear glasses out-of-doors complain that raindrops distort the images and make them unrecognisable. It may perhaps console them if we call their attention to the splendid interference phenomena visible in these same raindrops. All they need to do is to look at a source of light in the distance-a street-lamp, for example. A raindrop that happens to be exactly in front of the pupil becomes strangely distorted, a spot of light with extraordinary projections and indentations, with a border of very beautiful diffraction fringes in which colours, too, are distinguishable (Fig. 104, a). One remarkable thing about it is that the spot of light
remains in the same place even when the eyeglass is moved slightly to and fro. Another is that the
general shape and protruding curves of the spot of light seem, at first, to bear no relation whatever to the shape of the raindrop. The explanation is simple. Regard the eye as a small telescope forming an image of a source of light in the distance, and the drop of water as a group of small prisms held in front of the objective. It is then clear that each small prism refracts a group of rays laterally, independently of its position on the objective (provided it is still within the opening of the objective); the shape of the light-patch will depend, however, on the value of the refracting angle and on the orientation of each small prism. A drop of water extended vertically does indeed give a horizontal streak of light.
But now the diffraction fringes! These would not exist if the drop of water formed an accurate lens and so imaged the source of light exactly at a point, for then all parts of the light wave-front, since they left the source simultaneously, would arrive at the image together with no change of relative phase. But since the surface of the water is curved irregularly the refracted rays do not meet in a focus, but are enveloped by a
caustic (Fig. 104, b). In such a case one always finds that through a point in the neighbourhood of the caustic there pass two different rays which have traversed light-paths of different lengths; interference therefore occurs. On drawing the wave surface one finds a point of reversal giving a cusp; at any moment, therefore, there will always pass through a point T two wave-fronts with a definite difference of phase (Fig. 104, c).
Marcel Minnaert,
The Nature of Light and Colour in the Open Air (Dover Publications, 1954, translation H. M. Kremer-Priest, revision K. E. Brian Jay), §118,
Interference Phenomena in Raindrops (pp. 167-169). The Dutch original can be found online here:
https://www.dbnl.org/tekst/minn004natu01_01/minn004natu01_01_0011.php
