Q.Using the idea of Rayleigh scattering, explain why the clear daytime sky looks blue and why the sun looks reddish near sunrise and sunset.

Rayleigh scattering by particles much smaller than the wavelength of light varies as $I\propto1/\lambda^4$, so shorter-wavelength blue and violet light is scattered many times more strongly by the molecules of the atmosphere than longer-wavelength red and orange light. Looking at any part of the sky away from the sun itself, what actually reaches the eye is sunlight that has been scattered sideways out of the direct solar beam by these molecules; since blue is scattered so much more than red, the sky appears blue in every such direction. At sunrise and sunset, the sun is very low on the horizon, so its direct light must travel an unusually long path through the dense lower atmosphere to reach an observer, compared with its much shorter, nearly vertical path at noon; over this long path, almost all of the blue and violet components are progressively scattered away out of the direct beam, leaving mainly the poorly-scattered red and orange components in the light that finally reach …

State the $1/\lambda^4$ scattering law, then apply it separately to (a) light seen away from the sun (scattered light dominates) and (b) light seen direc …

• Explaining the blue sky by the sky itself being 'coloured blue' rather than by wavelength-dependent scattering.
• Forgetting that the sunset explanation needs the LONGER atmospheric path at low sun angles, not just 'more atmosphere in general'. …