CBSE Class 10 Science 2017 · Paper 31/1/C · Question 7

Scattering of light: when light falls on very small particles in its path (molecules of air, fine dust, smoke, tiny water droplets), part of it is sent off in various directions. This is called scattering of light. Why the clear sky is blue: the molecules of air and other fine particles in the atmosphere are smaller than the wavelength of visible light, and such particles scatter light of shorter wavelengths (the blue end of the spectrum) more strongly than light of longer wavelengths (the red end); red light has a wavelength about $1.8$ times that of blue. So when sunlight passes through the atmosphere, its blue component is scattered much more strongly than its red. This scattered blue light reaches our eyes from all directions of the sky, and the clear sky appears blue. (If there were no atmosphere there would be no scattering, and the sky would look dark.)