Why the Sky Colour is Blue?

What is Scattering of Light? Why does the Sky look Blue? Why do We see Seven Colours in a Rainbow? Why is it Blue and not any other Colour?
Why the Sky Colour is Blue

All the birds fly so high and so do the kites, but as they fly farther, you can’t see their colours. But no matter how far you see the sky, it always appears to be blue. You all know that the sky colour has been the same since the beginning of time. But do you wonder, why sky appears blue in colour? Are there any theories behind it or is it just nature? Continue reading to know more about this phenomenon.

1. What is the Concept of Wavelength?

The wavelength of seven colors of the rainbow: sky colour 1

The main concept of wavelength is that the highest-energy light has the shortest wavelength with high frequency, while the lower energy light has a longer wavelength with low frequency. And, the wavelength of light is the distance between two consecutive troughs or crests in a transverse wave or light wave. It represents the travelling pattern of any travelling light. Wavelength is usually expressed by the units of nanometres (nm) or micrometres (µm). (See What are the 6 Layers of the Earth?)

The colours of the sunlight and their respective wavelengths are as follows, from longest to shortest.

  • Red: 620 to 750 nm
  • Orange: 590 to 620 nm
  • Yellow: 570 to 590 nm
  • Green: 495 to 570 nm
  • Blue: 450 to 495 nm
  • Violet: 380 to 450 nm.

Now you know that the red colour has the longest wavelength. But if these seven colours enter the Earth, then why sky colour is only blue? (See What are Examples of Elements in Everyday Life?)

2. Why Sky appears Blue in Colour?

why sky appears blue in colour

In order to understand the concept of the sky being blue in colour, you need to understand the phenomenon of the scattering of the sunlight. You all know that the sun is approximately 150 million kilometres away from the Earth. And since the Earth is round, the sun strikes the surface at various angles, ranging from 0°, just above the horizon, to 90°, directly overhead. 

The sunlight you receive is of white colour which in turn is a mix of seven colours. This is literally all the colours of a rainbow. But unless the rainbow is visible, the sky colour does not show any trace of these seven colours. This happens because when sunlight enters the surface of the Earth, it gets dispersed in all directions due to the atmospheric particles. Compared to all the seven colours, the blue light gets scattered more since it has shorter and smaller waves. And this is why you get to see a blue sky. Also, check out how many colors are in the Rainbow?

3. What is Raleigh Scattering?

The diagrammatic representation of the Rayleigh Scattering phenomenon: sky colour 2

This phenomenon was named after a British physicist, John William Strutt popularly known as Lord Rayleigh, during the 19th century. The light from the sun disperses because it bends when it comes in contact with the gas molecules and other particle molecules in the atmosphere. So, when this happens, the light scatters. The seven colours scatter according to their wavelength, but the more the wavelength, the less that light will be scattered. The following is the formula used in this concept. Also, check out what causes double rainbow?

I=1/λ4

So, the colour of the sky is blue because the blue and violet lights are listed at the last in consideration of their wavelengths so they scatter the most after getting bent due to their contact with the molecules in the atmosphere. (See What Color does Red and Purple Make?)

4. What is Mie Scattering?

The diagrammatic representation of the Mie Scattering.

Just like Rayleigh scattering, it works on the principle of the shorter wavelength being scattered the most. Similarly, the Mie scattering is not affected or solely dependent on the lengths of the wavelength. (Also read What is the Main Source of Water?)

5. What is the Concept Behind Ocean Being Blue?

A beautiful sea coast with blue water and sky, some clouds and sun setting in the horizon.

You see the sky is blue because the colours blue and violet are scattered the most in the atmosphere. Similarly, when these light waves enter the sea or water bodies, then the light with the largest wavelength is absorbed by the sea. Hence, the red and orange mostly gets absorbed by the sea and the one with a shorter wavelength is seen. The absorption decreases as you go deep down the ocean, hence the blue colour fades or becomes less visible. Must read words to describe the Sea.

So blue is the sky colour and the reason behind it is discussed in this article. We hope that you now understand why sky appears blue in colour in spite of several other colours reaching the Earth in the form of Sunlight. (Also read How is Dew Formed?)

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