The blue colour of the clear sky is due to:

When sunlight enters the Earth's atmosphere, it strikes the tiny molecules of gases, such as nitrogen and oxygen, and other very small particles. These particles absorb and re-emit (scatter) the light in all directions.

According to Rayleigh's law of scattering, given by the British physicist Lord Rayleigh in 1871, the amount of scattering by very small particles is inversely proportional to the fourth power of the wavelength of light (scattering ∝ 1/λ⁴). This means that light of shorter wavelength is scattered much more strongly than light of longer wavelength. Blue light has a shorter wavelength (about 450 nm) than red light (about 700 nm), so it is scattered several times more. As a result, when we look at any part of the sky away from the Sun, we see the scattered blue light coming from all directions, and the sky appears blue.

Violet light has an even shorter wavelength than blue, but the sky does not look violet because sunlight contains less violet light, some violet is absorbed high in the atmosphere, and our eyes are more sensitive to blue.

Scattering also explains other phenomena:

• The Sun appears reddish at sunrise and sunset, because sunlight travels a longer path through the atmosphere, and most of the blue light is scattered away, leaving mainly red and orange light to reach our eyes.
• The sky appears dark (black) to astronauts in space, or at very high altitudes, because there is no atmosphere to scatter light.
• Danger signals are red because red light is scattered least and can be seen from a long distance, even in fog.
• Clouds look white because the water droplets are larger than the wavelength of light and scatter all colours almost equally.
• The Tyndall effect, the scattering of light by colloidal particles, makes a beam of light visible in a dusty room or through a forest canopy.

Why not the other options?

• ✗Reflection is the bouncing back of light from a surface, as in a mirror.
• ✗Refraction is the bending of light when it passes from one medium to another.
• ✗Dispersion is the splitting of white light into its seven colours, as in a prism or a rainbow.