Physics Q & A - The color of the sky
Lam Chun-hung (Translation by Wong Ka-lei)

Scattering of sunlight by the earth's atmosphere at sunset
Fig. 1   During sunset, sunlight passes through a thicker atmosphere in order to reach the ground, most blue light is scattered, leaving red light to reach the ground.
 
The sky appears red during sunset
Fig. 2   The sky appears red at sunset.

Why does the sky appear blue when it is sunny? Why does it appear red during sunset?

This is a consequence of the scattering of sunlight. When sunlight enters the earth atmosphere, air and water vapour molecules will absorb part of the light and reradiate it to all directions. This is called scattering. White sunlight is composed of light waves of different colors, whereas blue light has the shortest wavelength and red light has the longest. The short-wavelength blue light is easier to be scattered. The sun is very close to the horizon during sunset. Sunlight must pass through a thicker atmosphere in order to reach the ground (Fig. 1). Most of the blue light has been scattered away and red light remains. Therefore the sun appears red during sunset (see illustration). On the other hand, sunlight just has to pass through a thinner atmosphere during daytime, blue light is scattered less. Therefore the sun appears white. At the same time the sky is full of scattered blue light, that is why the whole sky appears blue.

A simple experiment of scattering

Equipment: 1 hand torch, a transparent glass fully filled with water, a little amount of milk.
  1. Add a few drops of milk into the fully filled glass of water. Stir the mixture very thoroughly.
  2. Isolate the setup from other light sources, use the torch to illuminate the glass.
  3. Observe the scattered light from different directions of the glass.
  4. Pay attention to the region near the light source, the milk suspension appears a little bit blue there, while it appears a little bit red in the regions far away from the light source. This shows that blue light is easier to be scattered.