| Which of the following fact about dust particles in the atmosphere is/are correct? |
| I. They act as hydroscopic nuclei around which water vapour condenses to produce cloud. |
| II. They produce dawn and dusk. |
| III. They have no role in the apparent colour of the sky. |
A) I and II
B) II and III
C) I, II and III
D) I and III
Correct Answer: A
Solution :
The blue colour of the sky must be due to small particles of dust and droplets of water vapour in the atmosphere. Even today, people sometimes incorrectly say that this is the case. Scientists realised that if this were true, there would be more variation of sky colour with humidity or haze conditions than was actually observed, so they supposed correctly that the molecules of oxygen and nitrogen in the air are sufficient to account for the scattering. The case was finally settled by Einstein in 1911, who calculated the detailed formula for the scattering of light from molecules; and this was found to be in agreement with experiment. The molecules are able to scatter light because the electromagnetic field of the light waves induces electric dipole moments in the molecules.
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