Why does the perceived colour of a tree-covered mountain change from green to indigo to violet as the distance of the observer from the mountain successively increases?
Sunlight consists of all visible wavelengths from violet to red. Earth’s atmosphere consists of gas molecules (78 per cent Nitrogen, 21 per cent Oxygen, 1 per cent Argon and traces of water vapour, hydrogen etc.) dust particles, smoke, water droplets etc.
The size of the gas molecules is very small (1000 times approximately) when compared to the wavelength of the visible light and hence the scattering of sunlight by these gas molecules is wavelength dependent. So, short wavelength radiations like violet and blue are scattered more (Rayleigh scattering).
When an observer sees a mountain under sunlight, he not only sees the light coming from the mountain (trees and plants) but also the sunlight scattered by the gas molecules of atmosphere (more correctly, troposphere) present between him and the mountain.
When he looks close to mountain, light from the mountain is more than the light scattered from the atmosphere. Hence, it appears green. However, when he looks at the same mountain at larger distances, length of the atmosphere is increased and also the intensity of light coming from the mountain is reduced.
As a result, light scattered by the gas molecules present in the atmosphere is more than the light from the mountain. So, it appears blue at long distances. Generally, blue colour appears predominantly over violet as it is more sensitive to human eyes.
One may ask what happens to the scattering of sunlight by dust, smoke particles present in atmosphere. The size of these particles is comparable or larger than the wavelength of the visible light. Scattering of sunlight by these particles has negligible wavelength dependence (i.e. all colours are scattered uniformly and hence appears white). However, this white light intensity is very small compared to blue light scattered by the gas molecules.
DR. K. SARAVANAKUMAR
Assistant Professor of Physics
Bannari Amman Institute of Technology
Sathyamangalam, Tamil Nadu