Name the natural phenomenon by which …

(i) Tyndal effect This is the scattering of light when it passes through a colloidal solution. The path of a beam of light passing through a true solution is not visible. However its path becomes visible when it passes through a colloidal solution. Where the size of particles is relatively larger. 

(ii) The colour of sky appears blue to us : During the day time, sky appears blue. This is because the size of the particles in the atmosphere is smaller than the wavelength of visible light, so they are more effective in scattering the light of shorter wavelengths (blue end of spectrum). When sunlight passes through the atmosphere, the fine particles scatter the blue colour more strongly than red. The scattered blue light enters our eye. Hence, the sky appears blue.

 (iii) At sunrise and sunset, colour of sun and sky appears red because of scattering of light. Near the horizon most of the blue light and shorter wavelength light are scattered away by the particles. Therefore light that reaches our eyes is of longer wavelengths. i.e., reddish colour.