In the previous section, we have discussed the atmospheric refraction. In this section, we will try to get the answer for the following question.


Why is the colour of the clear Sky Blue? 

Clear Sky

The air molecules and other fine particles present in the atmosphere are smaller than the visible light wavelength. These are more effective in scattering the light with shorter wavelengths at the blue end than those of longer wavelengths at the red end. The wavelength of red light is about \(1.8\ times\) higher than blue light. Thus, when sunlight travels through the atmosphere, the fine particles in the air scatter the blue colour more strongly than the red colour, which has shorter wavelengths. 


The scattered blue light reaches our eyes. There would not have been any scattering if the earth had no atmosphere. Then, the sky would have seemed to be dark.


The sky looks dark to passengers flying at very high altitudes because scattering is not prominent at such heights

You might have seen that ‘danger’ signal lights are red in colour

Do you know why? 

The red is least scattered by fog or smoke. Therefore, it can be seen in the same colour at a distance