CBSE Class 10 Science 2017 · Paper 31/1/1/C · Question 8

Scattering of light: when light falls on very small particles in its path, such as molecules of air, fine dust or tiny water droplets, the particles send the light out in many different directions. This spreading of light by the particles is called scattering of light. The clear sky is blue because 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) much 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 strongly in all directions, and this scattered blue light reaches our eyes from every part of the sky. Hence the clear sky appears blue.