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Look up! Just do it… What do you see? Stupid question, eh? You see the ceiling of course. And so… So far so good. When you look upwards during a sunny day while picnicking how ever, you see the blue sky. Ever wondered why the sky isn’t green nor but blue?

Before answering this question, lets go back to the basic physics of light. OK. Sorry, but here I go

Light is a kind of energy, which travels in waves. Clear? In actual fact, light is a wave of vibrating electric & magnetic fields. Light forms only one small part of a larger range of vibrating electromagnetic fields called the electromagnetic spectrum. Electromagnetic waves travel through space at an amazing 300 00km/s. Therefore this is also the speed of light.

When you see white light from the bulb, you’re actually seeing the seven colours of the rainbow! This is because white light consists of a combination of these seven colours.

Each of these colours has a different wavelength, frequency & energy. Wavelength is the distance between the crests of the waves. The frequency is the number of waves that pass by each second. Note that the longer the wavelength, the lower the frequency & the less energy it contains.

Violet: Shortest wavelength, highest frequency & energy

Red: Longest wavelength, lowest frequency & energy

Now back to our main topic. That blue colour of the sky is due to an effect known as the Rayleigh scattering. As light from the sun moves towards Earths atmosphere, not all the colours are actually able to pass through that atmosphere. Most of the longer wavelength colours do pass (red, orange & yellow for sure) but the shorter wavelength ones however are mostly absorbed by gas molecules found in the atmosphere. Absorbed blue light is then bounced back (radiated would be a better word) in all directions. This makes the shy to appear blue!

You would however ask me why its the blue rays, which get scattered, & not the indigo nor the violet rays (the other colours pass through the atmosphere remember theres also blue that pass through but it then gets scattered). Well in actual fact, the shorter the wavelength, the more the colour gets scattered. Violet, which has the shortest wavelength of all colours of the rainbow thus gets scattered more than blue rays do. Our eyes however are much more sensitive to blue than to violet & the sky therefore doesnt appear to be violet but blue (in reality the sky is violet-indigo!).

Now do take the time making the following:

Take a look at the horizon (if youre at the seaside) & note the skys colour there. No, its not as blue as the rest of the sky, is it? Its much paler. This is because for the blue light to reach your eye, it has to travel more. Blue light will thus pass through more air consequently getting scattered more. Conclusion: less blue light reaches your eyes than compared to blue light from just above you.

K.A.Cassimally is the editor in chief of Astronomy Journal & Astronomy Journal Ezine. He is also the co-founder of the RCPL Astronomy Club. K.A.Cassimally is best known for his article ‘Harry Potter & the Moons of Jupiter’.

He is also Senior Columnist at BackWash.com where he writes ‘Not Scientific Science’.

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K.A.Cassimally

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