Why the Sky Is Blue

The reason why the sky appears blue to the human eye

In order to answer this question, we need to know more about light, and the Earth's atmosphere.

The atmosphere surrounds the earth and is made up of a combination of gas molecules and various elements. It mostly contains the gases nitrogen (78%) and oxygen (21%) [1]; however Argon gas and vapor play a significant role as well. Amongst these, there exist small solid particles, such as dust, ashes, pollen, and salt.

Moving onto light, it is important to appreciate that it is an electromagnetic wave radiating energy. However, it is only a fraction of a wide range of vibrating electromagnetic fields, otherwise known as the electromagnetic spectrum. The part of this spectrum that our eyes can see is light with a wavelength range of 380-750 nm [2].

It is interesting how an everyday person believes that sunlight or the light from light bulbs is white, when in fact it is a mixture of many colours. By placing a prism in front a ray of light, we can deviate the light, and therefore reveal all the different colours, as we can naturally see in the case of rainbows [3].

White light is made up Red, Orange, Yellow, Green, Blue, Indigo, and Violet respectively, [4] where violet has the highest frequency and red has the lowest. This consequently means that violet has the highest energy, and red the lowest as indicated by the direct proportionality in Planck's Equation E = hf [5] (where h is Planck's constant, f is the frequency of light and E is the energy).

Light will travel in a straight line until it hits some dust or a gas molecule that is in its way. Depending on the wavelength of the light and the size of the object it hits, it will behave accordingly. In the case of the sky, the wavelength of visible light is greater than the size of a gas molecule. Therefore when a gas molecule is being hit by light, the molecule absorbs some of it and then radiates it in a different direction but in the same color as that of the absorbed light.

This is not the case with all colours though. All seven colors can be absorbed, but we find that molecules absorb higher frequencies more frequently than lower frequencies. This process is known as Rayleigh scattering [6].

So, why is the sky blue?

When light moves through the atmosphere, the majority of the longer wavelengths are not absorbed, therefore their path is not disturbed. Some of the red, orange, and yellow light behaves the same since it is barely affected by the air. On the other hand, the gas molecules absorb most of the shorter wavelengths (blues) and then radiate them all over the sky.

Hence, wherever you look, there will be scattered blue light reaching your eye as shown in the diagram below.