The sky is blue. But, Why is the sky blue? It’s a question that almost everyone has pondered on. It’s due to a phenomenon called scattering of light (Rayleigh scattering).

Also, the red light is used in danger signals and the tail lights of vehicles are red due to the same reason. So, let’s take an overview of this phenomenon which takes us to our goal of understanding this blue appearance as well as the use of red lights for different purposes.

### The color composition of white light

The light that makes everything visible, the ordinary white light as we say can be split into seven different colors and this splitting process is called dispersion of light. Rainbow is the best example of dispersion of light I guess. The colorful soap bubble, the colorful lines seen as light passes through a prism, etc. are some examples of dispersion in everyday life. The colors in which white light is dispersed are red, orange, yellow, green, blue, indigo, and violet respectively in the decreasing order of wavelength.

A simple experiment can prove that these colors form white light. Take a circular disc and divide it into seven segments as in the figure. Color each segment with each color as mentioned above and rotate the disc at high speed. You won’t see colors in the disc, just everything that appears white.

### Scattering of light (Rayleigh Scattering)

Small particles, so small that the size is comparable to the wavelength of light (350 – 780 nanometers i.e in the range of 10,000th fraction of a millimeter) or even smaller can scatter the white light. This scattering process is called Rayleigh scattering.

Rayleigh stated that the degree of scattering is inversely proportional to the fourth power of wavelength. So, the component colors are scattered according to their wavelengths. The violet has the shortest wavelength and is scattered more while red light has the longest wavelength and is scattered less.

Since red light is scattered less, it is used in danger signals, tail lights of vehicles, etc. to avoid scattering. This can help observers to see the light clearly at a longer distance because red light can travel long distances without getting scattered.

### Why is the sky blue?

Our atmosphere contains different gases in a molecular form whose size is even smaller than the wavelength of visible light. As the light coming from the Sun enters our atmosphere, it encounters a lot of such particles. Then the light gets scattered on the basis of the wavelength as stated above. The scattering suffered by red light is less while violet light is more.

The scattering of blue light is slightly lower than violet but more than green, yellow, and red light. The scattering process eventually results in the sky being blue since blue light is scattered much due to its shorter wavelength.

### Why isn’t the sky violet?

Although violet and indigo light has a shorter wavelength than blue light, they are not seen. This is because the content of these lights is low in the sunlight inside the atmosphere since light with a higher wavelength is absorbed by a higher atmosphere. Also, our eye is much more sensitive to blue light than violet and indigo so the blue light is easily visible. So the content of blue light and our eye’s sensitivity toward blue light makes blue light superior to violet and indigo.

### Why is the sky red during low Sun?

During low sun (dusk and dawn), the light rays coming from the Sun have to travel more distance throughout the atmosphere. So, the rays will encounter more particles which increase the scattering of light much further. This results that the blue and other color light with lower wavelengths scattering too much and away from our line of sight. So, only less scattered light i.e the light with a longer wavelength can reach us. This means light having a longer wavelength – yellow, orange, and red light can reach us. Eventually, the sky appears yellowish-red during the low Sun.

The color change of the sky is just due to the scattering of light due to the molecular particles present in the atmosphere. So, if we view the sky from above the atmosphere is dark. We cannot see any colors then.

Ashwin Khadka is a PhD Scholar in Nano Energy and Thermofluid Lab in Korea University, Republic of Korea under Korean Government Scholarship Program. He has a Masters Degree in Physics from Tribhuvan University, Kathmandu, Nepal. He is a science enthusiast, researcher and writer.