Rainbow formation
Since clouds contain tiny water droplets, why are rainbows not permanently present?
Kshitij Rahalkar, Chhattisgarh
ANSWER: On a rainy day, sunrays, which are polychromatic (with all the seven colours), fall on water droplets at a certain incident angle and refract with certain other refractive angle because water is denser than air.
Since the line of incidence of the rays may not be collinear to the drop's diameter, the angle of refraction, however, is different for different colours of the rays in order that each colour catches up with the other at the opposite side of the droplet (remember the frequency of a given colour is invariable irrespective of the medium and the velocity of all colours is same in a given medium.)
In other words, a polychromatic ray gets dispersed into seven colours of the visible light as the light beam is refracted into the body of the droplet.
The colours, thus dispersed undergo total internal reflection on the opposite inner side of the droplet and reach the eye to enable us feel the virtual image of the rainbow. (In fact, the mechanism of appearance of the rainbow is more complex than this and there is a cooperative phenomenon that includes interference in space and time.)
However the extent of the resolution of the seven colours in the droplet depends greatly on the extent of the (path) length the rays cover in the droplet.
Consider the case of seven sprinters of varied speeds covering a shorter (say 100 mts) and a longer track (say 400 mts).
In a shorter track the relative gap, after the run is less whereas in a longer track it is more.
In normal clouds, the droplets are too small to cause sufficient resolution of the colours and to enable total internal reflection and hence rainbow is invisible with dry clouds.
On a rainy day, the droplets are big enough to cause the resolution and total internal reflection of the colours to enable us to see the rainbow(also they are numerous and closer and at an appropriate viewing angle to allow the cooperative phenomenon).
Prof. A. Ramachandraiah
Convener S&T, Jana Vignana Vedika
Warangal, Andhra Pradesh
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