OPTICS OF THE RAINBOW AND THE PHYSICS OF RAIN 
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Fic. 6—The natural oscillations of raindrops; part of an enlarged photograph 
(which were about 6 mm in radius) were 
strongly damped, decaying to 1/10 amplitude 
in one second. Moreover, Blanchard [1950] re- 
cently found other types of oscillations in drops 
suspended in an upward-directed jet, which was 
presumably turbulent. He and Lenard roughly 
confirmed the theoretical law of Rayleigh [1879] 
(Fig. 5) 
1 / Qo 
Ds = ,/ | ee 
tg 4/ b6r3 
(=3.84r-3? for distilled water) 
where a = surface tension, and p = density. 
Here we may recall two earlier observations 
by Poey [1863] and Laine [1909]. These observa- 
tions noted that the rainbow (particularly the 
more sensitive secondary bow) showed vibrations 
with each peal of thunder. It is quite likely that 
this indicates droplet oscillations excited by the 
intense sound waves. 
Almost nothing is known however about the 
oscillations of freely falling raindrops. Earlier, 
Lenard [1887] and Schmidt [1913] carried out a 
few experiments at night but they recorded only 
a very small fraction of drops in oscillation. 
Some preliminary results of a new investiga- 
tion indicate that most of the larger raindrops 
are oscillating. If the fallimg raindrops are il- 
luminated with a reflector bulb, and we look in 
the direction of the divergent beam, observations 
or photographs (Fig. 6) near the rainbow angle 
show bright traces of raindrops. Oscillating drops 
show traces broken by equidistant dark spaces. 
The distance between dark spaces (s = v;/y, 
where v, is the fall velocity) les visually be- 
tween 0.5 and 10 em, and photographically be- 
tween 2 and 10 em. Using Rayleigh’s formula 
this gives droplet radii between 0.4 and 1.4 mm 
or between 0.6 and 1.4 mm for photographic 
images. Analysis of a photograph taken in 
shower rain of medium intensity, gives a radius 
distribution for the oscillating drops (Fig. 7, 
top). 
There are many physical problems about these 
oscillations which have yet to be solved. Are the 
oscillations maintained by eddies generated in 
