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Sci Tech
Cloud colour
While water is colourless, why are clouds formed of water white?
Arjit Raj
Port Blair, Andaman & Nicobar Islands
On incidence of white light on an object, some light gets absorbed and some gets scattered or reflected. Depending on the colour or colours of light it reflects or scatters, the object appears to have specific colour.
If white light is completely reflected as from a smooth surface, the object would appear white even if the object bears a particular colour in the light transmitted though it.
Take for example, the surface of a mass of colourless water. If you look at it in a direction so as to receive the sun light reflected from the surface the water mass would appear white. In fact, with ripple on the surface, the spots reflecting the light would appear white while the other places would appear transparent.
Because of the fact that water is transparent to white light, it does not absorb light of any colour; so the spots of the water surface appear white.
A similar observation is experienced while looking at the water spray of a fountain or garden spray in sun light, where some of the droplets appear white while others transparent.
Similarly and for the same reason crushed fine pieces of transparent glass or sugar appear white and opaque. The cloud is a collection of tiny water droplets formed by condensation of water molecules in the atmosphere. Because of the highly curved surface the droplets have, sunlight gets reflected from their surface in all directions.
Another phenomenon also takes place; the light rays can penetrate the water droplets at different angles and due to difference in the refractive index for light of different colour, white light splits to light rays of different colour.
This process is known as dispersion of composite light. This happens from all the droplets and in all directions. Thus the net effect is the combination of all the colours resulting in the appearance of the white light.
However, when the droplets are larger in size and less in number, the dispersed light of split colours appear in a regular arrangement of the rainbow.
Further, because of multiple reflection of the dispersed light inside the droplet, the rainbow appears in a different location than that of the droplets. But, as the white cloud consists of very fine droplets in high number density, the dispersed light recombines to white light which along with the reflected light make the cloud appear white.
H. K. Sahu
Scientific Officer
Indira Gandhi Centre for Atomic Research
Kalpakkam, Tamil Nadu
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