CBSE Class 10 Science 2017 · Paper 31/2/2 · Question 17

Scattering of light: when light falls on very small particles in its path (molecules of air, dust, smoke, water droplets), part of it is sent out in different directions; this is called scattering of light. Very fine particles scatter mainly the shorter wavelengths, at the blue end of the spectrum. Blue sky: the molecules of air and other fine particles in the atmosphere are smaller than the wavelength of visible light, so they scatter blue light much more strongly than red light (red has a wavelength about 1.8 times that of blue). The scattered blue light reaches our eyes from all parts of the sky, so the clear sky appears blue. Reddish Sun at sunrise: near the horizon, sunlight passes through much thicker layers of air, a far longer path, before reaching us. On the way most of the blue and other shorter wavelengths are scattered out of the beam, and the light that reaches us is mainly of longer wavelengths, so the Sun appears reddish.