Why is the sky blue? Today in VERITAS we attempt to answer this question which we have all asked as children.
We will need to explain three colours: The blueness of the sky, the redness of sunset and the whiteness of the sky that is near the sun.
THe atmosphere is made up of oxygen and nitrogen molecules. These molecules interact with the light that comes from the sun. These molecules absorb the light and then emit it. The light that comes to us from the sun is absorbed. So the absorption is direction specific( it happens to the light that comes to us from the direction of the sun). However after absorption of light the atom may emit this light in any direction. This phenomenon is called scattering of light.
Scattering of light is a complex phenomenon and cannot be fully understood without the help of modern Quantum Mechanics. And there are several different kinds of scattering depending on the properties of the incoming light and the atoms. The most prominent scattering phenomenon that occurs in the sky due to nitrogen and oxygen molecules is called Rayleigh scattering. In this the different colors of the light from sun are scattered differently. The scattering is inversely proportional to the 4th power of the wavelength of light that strikes the atom. What this means is that a light with smaller wavelength is scattered much much more that the light of a larger wavelength.
We know that blue has a shorter wavelength and red has the longest wavelength. So the blue light is scattered away by the molecules. Blue light is scattered about 10 times more than red light. That means that blue light is splashed all over the canvas of the sky. The red color however more or less maintains its incoming direction. So whereever you see you will see blue in the sky. The color of blue would be deeper the further you look from the sun.
But why isnt the color of the sky violet when we know that violet has the least wavelength? Should it not get scattered everywhere and color the sky? Yes! Violet gets scattered even more than blue. But our eyes are more sensitive to blue than violet. And sometimes if you look carefully you would see a tinge of violet in the sky.
What happens during sunset. The sun ‘s direction is at the other end of the atmosphere. The rays of the sun have to travel a larger distance to reach us and they encounter more of the molecules on the way. So when we look at the sun we look at the light that has been least scattered. The light that has been most scattered would appear in the color of the sky. The light that has been least scattered appears in the direction of the sun. What light is this? Red of course! (max wavelength = least scattering). That is the reason why sunsets appear red in color.
Now lets try to explain the whiteness of the sky near the sun( except at sunset). This is explained by anothet kind of scattering called Mie scattering. Mie scattering is not wavelengh dependent. All wavelengths get scattered equally and the direction of the scattering is mostly in the forward direction( the direction of the incident light). So when we look at the sun we will see the forwaded light of the Mie scattering and it is a mix of all possible colors( white).
A sense sublime
Of something far more deeply interfused,
Whose dwelling is the light of setting suns,
And the round ocean and the living air,
And the blue sky, and in the mind of man.
Does this answer all our childhood questions. Yes some of them.. but it poses many more : ” What if I go to another planet… would the sky be blue? or some other color? What if the atmosphere was made of some other gases? …. “
Isnt is more fun when you carry your childhood curiosity with you?
Go, wondrous creature! mount where Science guides:
Go, measure earth, weigh air, and state the tides:
Instruct the planets in what orbs to run,
Correct old time and regulate the Sun;