Thursday, July 31, 2008

How strange! The sky is blue!

Author: K.A.Cassimally

Look up! Just do it. What do you see? Stupid question, eh? You see the ceiling of course. When you look upwards during a sunny day while picnicking however, you see the blue sky. Ever wondered why the sky is not green nor but blue?

Before answering this question, let's go back to the basic physics of light. Sorry, but here I go…

Light is a kind of energy, which travels in waves. In fact, light is a wave of vibrating electric and 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 are 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 and 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 and the less energy it contains.

·Violet: Shortest wavelength, highest frequency and energy ·Red: Longest wavelength, lowest frequency and 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 Earth's atmosphere, not all the colours are actually able to pass through that atmosphere. Most of the longer wavelength colours do pass (red, orange and 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 it's the blue rays, which get scattered, and not the indigo nor the violet rays (the other colours pass through the atmosphere remember – there's also blue that pass through but it then gets scattered). Well in 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 and the sky therefore doesn't 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 you're at the seaside) and note the sky's colour there. No, it's not as blue as the rest of the sky, is it? It's 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.

About the author: K.A.Cassimally is the editor in chief of Astronomy Journal and 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 and the Moons of Jupiter'. He is also Senior Columnist at where he writes 'Not Scientific Science'.


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