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 red 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 ( our own GlamKitty explains it here),
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
000km/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 sky 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
right pass through the atmosphere remember - there's also blue that
passes through but it then gets scattered). Well in fact, the shorter
the wavelength, the more that 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 observation: 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.

PS: If you're not convinced with my explanation, maybe this one
below will.

Why the Sky is Blue

I don't suppose you happen to know

Why the sky is blue? It's because the snow

Takes out the white. That leaves it clean

For the trees and grass to take out the green.

Then pears and bananas start to mellow

And bit by bit they take out the yellow.

The sunsets, of course, take out the red

And pour it into the ocean bed

Or behind the mountains in the west.

You take all that out and the rest

Couldn't be anything else but blue.

Look for yourself. You can see it's true.

- John Ciardi

Other science issues (not too complicated don't you worry)
can be found at:

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