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Why is the Sky Blue? | Don't Memorise

Some of the common questions that kids ask are,

“why the sky is blue while the clouds aren’t?”,

“why is the sun yellow?”

and “why does it turn reddish orange during sunrise and sunset?”

These questions might look difficult to answer at first.

However the physics behind them is not difficult at all!

There are two things that play an important role

in giving a colour to the sky.

One is the atmosphere of the Earth

and the other is the sunlight of course.

So we need to know a more about these two first.

The white light coming from the sun is a mixture

of seven primary colours ,

and each coloured light has a corresponding frequency

and wavelength associated with it.

The violet coloured light has the shortest wavelength of all

while the red one has the largest wavelength!

And what forms the Earth’s atmosphere?

The Earth’s atmosphere is mainly composed of gases,

dust particles, water droplets and water vapour.

Among the gases,

Nitrogen is present in abundant quantity,

at around 78%.

It is followed by Oxygen which is approximately 21%

and the remaining is filled with the likes of Argon,

Carbon dioxide and other gases.

First let’s see what happens when the sunlight falls

on these air molecules.

When sunlight strikes these molecules,

it gets scattered.

Scattering is a process

in which the light is absorbed by the atoms

and re-emitted back in various directions.

However not all the light is scattered equally!

The amount of light that will be scattered

is given by the Rayleigh law of scattering .

It says that the amount of scattered light

or the intensity of the scattered light

is inversely proportional

to the fourth power of the wavelength of a wave.

‘I’ is inversely proportional to ‘lambda to the fourth power’!

If the value of lambda increases, then the intensity will be lower.

If its value decreases,

then the intensity of scattering will be more!

Lesser the wavelength, more will be the scattering!

So now I want you to tell me,

among the seven colours of sunlight,

which one will be scattered the most?

Yes,

the light of the colour corresponding to the smallest wavelength

will be scattered the most!

So blue, indigo and violet light

will be scattered most by these molecules of air!

And when this light travels to our eyes,

the sky appears blue to us!

But wait,

the violet light has the smallest wavelength!

Then why doesn’t the sky appear violet or even indigo for that matter?

It is true that sky scatters the violet light more than blue light.

However, we don’t see violet sky, do we?

What’s the reason?

The following graph tells us

how much of a particular wavelength is emitted by the sun.

Observe the graph well.

As you can see,

the sun emits more blue and light-bluish light

compared to the violet light!

Hence more blue light is scattered by the air molecules

resulting in the blue colour of the sky.

And another reason why we cannot see violet light

is because our eyes are more sensitive to blue light

than to violet light.

So this was about how the gases present

in the atmosphere scatter light!

Do dust particles and water droplets also scatter light?

Yes they do!

They scatter light as well.

However their size is comparatively

much bigger than the size of the light waves.

And hence they scatter all the colours of light in equal amounts!

So the white light which is entering the cloud or the dust particles,

emerges out as white light again!

Hence the clouds appear white!

There’s one last question we need to answer!

Why does the sun appear reddish during sunset

and yellow otherwise?

We will see that in our next lesson.