Take a look at the picture on the
screen.
Over here, you can see the sky. Now
we have studied that when the Sun gives
out light, it gives out white light. But from
your day-to-day observations, you have
seen that the sky appears blue in color. Now
if the Sun is giving up white light, why
does the sky appear blue in color?
Why does it not appear white in color?
We shall soon find out.
Before we get to the depth of this
interesting mystery, let us consider a
very mind-boggling fact. Did you know
that the distance of the Earth from the
Sun is a hundred and fifty million
kilometers?
This distance is such a huge distance
and it is the distance which the light
rays have to cover in order to travel from the 
Sun to the earth. That is a hundred and fifty
million kilometers.
Now we have studied the Sun emits all
kinds of electromagnetic waves. It emits
gamma rays, X-rays, UV rays, visible
light that consists of seven components
and also infrared waves, microwaves and
radio waves. Now all this together
are emitted by the Sun. Now what do you think happens 
once they reach earth? If you recall from our 
earlier discussions, you will remember
that we human beings are unable to
perceive infrared rays, microwaves, radio
waves, UV rays, X-rays and gamma rays. Now
even though in infrared rays, microwaves and
radio waves enter the earthís atmosphere, we are
unable to perceive them. And the earth's
atmosphere blocks out most of the UV
rays and almost all of gamma rays and
x-rays. But the earth's atmosphere letís
in visible light.
And as we have studied human beings can
perceive visible late that consists of
seven colors. Now let us find out what was
happening in the case and why the sky is
appearing blue.
Consider an animation.
We have learned previously that all
colors in the visible light spectrum, can
be composed of three primary colors.
These three primary colors are red, green
and blue. So using red, green and blue we
can derive any color based on their
relative composition. So here we consider
red light, green light and blue light
incident on the edge of the earth. Now let
us find out what happens after they are
incident.
Now before we consider that, look at the
animation.
In this animation,  rope has been tied to
the tree and the other end of the rope
is held in a person's hand. Nnow when the
person moves up and down less vigorously,
longer wavelengths are created. As a
result, the number of waves passing
through at any given point at a time, is
less. So it is hitting the mango, or 
the orange on the tree less.
Similarly, if the person on the other end
is jerking that rope up and down more
vigorously, wavelengths of shorter
lengths are created. And the number of
waves passing through any given point at
any particular time is more. And as a
result, the orange or  mango on the tree is
getting hit more number of times.
Now scientist Rayleigh had discovered a very
important observation. Lord Rayleigh, as he was
called, discovered that when the
wavelength of a particular wave is less,
it scatters more.
And when the wavelength of a particular
wave is large the scattering of that
wave is less.
This is the relation that Lord Rayleigh
had found out. And he had also observed a
scientific or mathematical relation
along with it.
The mathematical relation stated that
intensity of scattering is inversely
proportional to the fourth power of wavelength. 
Or in other words the intensity of
scattering is directly proportional to
one by lambda to the power four. Here lambda is
the wavelength of the wave that is being
considered.
So now we consider these three waves. That is, the three
different colors red, green and blue. Now
from our earlier discussions you will
recall that red had the highest wavelength,
followed by the wavelength of
green; and lastly the wavelength of blue.
So among these three colors, red has the
highest wavelength, followed by that of
green,
and then at the end we have blue.
So red has the highest wavelength
and blue has the lowest wavelength .
So here we see that red light scattered
very less than passing through the
atmosphere. Just as Lord Rayleigh had put
forward this relation, red light which
has a very large wavelength is scattered
to a very very less extent, and we can
see that is almost all scattered. Because
the wavelength of red ladies maximum. It
easily passes through the dust particles and
molecules that are present in the
atmosphere. As the result, it is very less
scattered . If we have to judge the
situation with the help of the formula,
we can say that since the wavelength of
red light is much more, the denominator in
this case will be very high. So this and
an expression will be very low. And as a
result, the intensity of scattering is
very low.
Similarly, we consider the case for green light.
Green light has a wavelength that is
less than that of red light. So wave so,
since the wavelength of green light is
less than that of red light. According to
what Rayleigh had said, it will be scattered
more. So as you can see, as compared to
red, green is getting scattered more. It
is actually colliding with the dust
particles and the molecules that are
present inside the medium. That is, it is
getting reflected off their surfaces and
getting scattered.
So it is scattered more than red light. Likewise, 
if we have to judge the situation based on
the formula,
we can ay that the
of green is less as compared to that of
red.
So this fraction will evaluate to a value
that is more than that for red. So we can
say that the intensity of scattering is
more for green than it was for red.
Lastly if we consider blue light, we can see
that it is getting scattered to a very
large extent. Why? Because the wavelength
of blue light is the least among red,
green and blue. So since the wavelength
of blue light is very less than that of
red, that is, it is the minimum in case of the
three, it collides with the dust particles
and molecules and it is scattered the
most, or to a very large extent. Similarly,
if we have to judge this situation based
on the formula, we can say that
since the wavelength of blue light is 
minimum or is very less,
the denominator is very less as compared to
the other two cases and the entire
fraction is more as compared to that for
red or even green. So the intensity of
scattering for blue is much more than
that of green or red.
So when red, green and blue light are
passing through the earth's atmosphere,
red passes by almost without scattering
or very little amount of scattering.
Green light on the other hand undergoes a
greater amount of scattering. But blue
light as we saw, because it has the
longest wavelength undergoes the maximum
amount of scattering. So after scattering
in the earth's atmosphere, this blue light
reaches earth. And on earth, it reaches our eyes. 
So it is due to this season that the sky or the
atmosphere seems blue to us.
Now you must be wondering that when you
see movies which deal with outer space,
or when you see astronauts walking on the
moon,
why do think that the astronauts are
able to see the blue sky? This is because
the moon or the outer space has no
atmosphere. There is complete vacuum
there and there are no medium particles.
So when sunlight is falling over
here, there is no scattering. And as a
result, no colors reach the astronauts
eyes and it is due to this reason that an
astronaut sees the sky around him as
completely black. And it is also due to
this reason that in movies, you see that the
color of the sky in outer space is
depicted as black.
Now you must have observed that
when the Sun is rising in the morning
or setting in the evening, the color
of the sky remains no longer remains blue. It 
becomes a very deep hue of orange and red .So why
do you think this happens? We just
discussed why the color of the sky was
appearing blue and not red. So now, why
is it appearing red and not blue?
Let us see.
During sunrise or sunset, the position of
the earth that this at that particular
point on the earth, where sunrise or
sunset this taking place is such that
the distance from the earth to the Sun
is maximum. That is, during a day the
distance from the Sun to earth is
maximum during sunrise and sunset. So as
 result, light has to travel very long
distance in order to reach earth.
Now we have studied that blue light is
scattered the most, green light is
scattered relatively less than blue light
and red light is not scattered at all or is
scattered very less.
Now since it has to travel a very long
distance, you can see from the animation
what is happening. When blue light is
travelling such a long distance, it is getting
scattered and not reaching the surface
of the earth from the atmosphere. On the other
hand, green light is also getting scattered
but not as much as blue light.
And lastly we see that red light is
passing through the atmosphere without
any scattering and is reaching the
surface of the earth. So it is due to this
reason that at sunrise and sunset, we are
able to see the color of the sky as red.
