The sky is blue. But, Why is the sky blue? Its 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 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.

### Color composition of white light

The light that makes everything visible, the ordinary white light as we say can be split into seven different colours and this splitting process is called dispersion of light. Rainbow is the best example of dispersion of light I guess. The colourful soap bubble, the colorful lines seen as light passes through a prism, etc. are some examples of dispersion in everyday life. The colours 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 colours form white light. Take a circular disc and divide it into seven segments as in the figure. Colour each segment with each colors as mentioned above and rotate the disc in high speed. You won’t see colours in the disc, just everything 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 millimetre) 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 colours 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 distance 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 by violet light is more.

The scattering of blue light is slightly lower than the violet but more than green, yellow and red light. The scattering process eventually results in the sky to be 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 of 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 higher wavelength is absorbed by higher atmosphere. Also, our eye is much sensitive to blue light than violet and indigo so that the blue light is easily visible. So the content of blue light and our eye’s sensitivity towards blue light makes blue light superior to violet and indigo.

### Why is sky red during low Sun?

During low sun (dusk and dawn), the light rays coming from the Sun has 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 colour light with lower wavelength to scatter too much and away from our line of sight. So, only less scattered light i.e the light with longer wavelength can reach us. This means light having longer wavelength – yellow, orange and red light can reach us. Eventually, the sky appears yellowish-red during low Sun.

The colour 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 its dark. We cannot see any colours then.