As the sun’s light passes through the atmosphere, it’s rays are scattered by tiny particles of pollen, soot, and dust to be found there. As blue light is scattered most, this is the reason why the colour of the clear sky is blue. At sunset and sunrise, sunlight has to travel more further to reach us. As the blue light has been absorbed by the atmosphere, so only red light can be seen.
This is the simplest answer to explain this phenomenon. However, the physics behind it is quite easy to understand. Let’s us dive deep into physics and learn 5 reasons why the colour of the clear sky is blue?
Two most important things give colour to the sky. They are-
- The atmosphere of the earth
Let’s learn the role os these two things first.
Visible light consists of a range of spectrum of different wavelengths. Light with the longest wavelengths are at the red end of the spectrum, and light with shortest wavelengths are at blue and violet end. The white light coming from the sun is a mixture of seven primary colours. Each coloured light has a corresponding frequency and wavelength associated with it. The violet coloured light has the shortest wavelength of all while the red light has the largest wavelength.
2. The atmosphere of the earth
The earth’s atmosphere consists of tiny dust particles, water, droplets, pollen, soot and water vapour. Among the gases, Nitrogen and Oxygen are the most abundant gases present in the earth’s atmosphere. Nitrogen accounts for 78% followed by oxygen which is approximately 21% of all the gases present on earth’s atmosphere. Remaining 1% is present with the likes of Argon, carbon dioxide and other gases.
Since the blue light has shorter wavelengths, they are more likely to strike those tiny molecules of oxygen and nitrogen more often, and when they do, they get scattered in all directions.
What happens when sunlight comes in contact with the atmosphere?
When sunlight enters the earth’s atmosphere it falls on those small molecules and gets scattered.
Scattering is a process in which air molecules absorb sunlight and re-emit it in various directions.
However, not all the sunlight is scattered with equal intensity. The amount of light that will be scattered is determined by Rayleigh’s law of scattering.
Rayleigh’s law of scattering
The amount of scattered light or the intensity of scattered light is inversely proportional to the fourth power of the wavelength of a wave.
If the value of ^ increases, then the intensity will be lower if it’s value decreases then the intensity increases.
In simple words, more the wavelength lesser will be the scattering & lesser the wavelength greater will be the scattering. The light having the lowest wavelength will be scattered most.
Blue, Indigo and violet light have the lowest wavelengths so these colours will be scattered most by the molecules of air and when these lights fall on our eyes the clear sky appears blue to us.
Among these three colours, violet has the lowest wavelength, then the question arises why does the clear sky not appear violet or indigo for that matter?
According to the mathematical proof by Rayleigh’s scattering, blue wavelengths in the atmosphere are scattered 16 times more than the red wavelengths.
Why the colour of the clear sky is blue and not violet?
Though violet light indeed scatters more than the blue light, however, we don’t see the clear sky in violet, do we?
The above graph shows the intensity of the wavelength of light emitted by the sun. As you can see that the sun emits more light blue colour and light-bluish light more compared to the violet light. Hence, the intensity of blue light scattered by the molecules is more than the violet light, this is another reason why the clear sky has blue as the dominant colour. Another reason why we cannot see violet light is that our eyes are biologically more sensitive to blue light than to violet light.
Retina structure of the human eye
The retina of the eye consists of layers of tiny cells called rods and cones. These cells contain coloured that react when light falls on them, triggering a nerve impulse.
Rods are slim cells that are responsible for black and white vision. Cone cells are responsible for colour vision. They contain different light-sensitive substances that respond to either red, yellow-green or blue-violet light. Combined with the black and white images produced from the rods, cone cells give you the coloured picture that you see. The cone cells only work in bright light which is why it is difficult to see colours in dim light.
Do dust particles and water droplets also scatter light?
Yes, they do. Dust particles and water droplets also scatter light. However, their size is comparatively bigger than the size of light waves. Hence, they scatter all the colours of light in equal amounts. So the white light which is entering the cloud or the dust particles emerges out as white light again. This is the reason why clouds appear white and the clear sky appears to be blue.