Scattering of Light
In a recent class demonstration a narrow intense beam of red laser
light was shined across the front of the classroom. Neither
the students nor the instructor were, initially, able to see the
laser beam. It was only when some chalk dust or a cloud of
microscopic cloud drops fell into the laser beam and scattered
some of the light that we were able to see it. Because of
how the laser beam was able to pass invisibly through air you were
probably left with the following conclusion
Well that's not entirely true. Even something as small as
air molecules can scatter light. We weren't able to see the
laser light being scattered by the air because the scattered light
was too dim. If the laser beam had been more intense or if
we could have added more molecules to the air in the room we might
have been able to see laser light being scattered by the air.
Sunlight is very intense and when it shines through the whole
atmosphere, where there's lots of air, we are able to see the
scattered sunlight. The atmosphere also contains
particulates and clouds and they scatter sunlight.
We going to consider all of this now. But first we'll look
at what would happen, what you would see if the earth didn't have
If you went outside and looked at the sun (you
shouldn't do that of course) you'd see a bright sun against a
black background. You'd see the sun because you're
looking back in the direction of one of the rays of light
coming from the sun.
It would be like putting your head at the end of the laser
beam in the demonstration the other day and looking back
toward the laser.
If you look away from the sun and toward the sky you
wouldn't see anything. The sky would appear
black. That's because there's nothing to scatter or
redirect the sunlight. This is just like when you
couldn't see the laser beam as it traveled across the
front of the classroom. You couldn't see the beam
unless something was put into the beam to scatter some of
the laser light.
To give you some sense of what black sky would look like,
here's a famous photograph of earth taken on Christmas eve
(Dec. 24, 1968) by the Apollo 8 astronauts as they orbited
the moon (source
of this image).
sun is obviously shining in the picture, we can see it
being reflected by the surface of the moon and the earth
in the distance. Sunlight is also shining through
space between the moon and the earth but there's nothing
there to scatter the light. So you just see black;
that's what the sky would look like if the earth didn't
have an atmosphere.
Before we take the next step we need to remember that
white light is actually a mixture of violet, blue, green,
yellow, orange and red.
In the next picture we'll add an atmosphere. Just
air molecules, no particles, or clouds.
But there's something new. In the class demonstration
we used just a single color of light, red. The incoming
sunlight is white, a mixture of all the colors. Air
molecules don't scatter all the colors equally, they scatter
the shorter wavelengths (violet blue green) in greater amounts
than the longer wavelengths (yellow orange red). This is
depicted above. Actually the figure shows only short
wavelengths being scattered just to keep things simple.
Air molecules scatter light in this way because they are very
small (much smaller than the wavelength of visible light).
Violet has the shortest wavelength and is scattered the
most. However there isn't as much violet in sunlight
as there is blue and green. There's a lot of green
light in sunlight (more than any other color as a matter
of fact) but it isn't scattered as readily as blue.
So the end result is that we see blue light coming from
the sky. This is why the sky is blue. When the
air is clean (from of particulates), the sky has a deep
blue color. The
response of our eyes is also involved, here's a
little more explanation.
We were careful not to look directly at the beam of laser light
in the class demonstration. It's too intense and could
damage our eyes. The same is true of the sun. We don't
look directly at the sun. It is safe to look at the
scattered light coming from the sky. It is much weaker, only
a small portion of the beam of intense sunlight is scattered.
In our next picture, we'll add a cloud to the
picture. As we saw in the laser demonstration, cloud
droplets and ice crystals are good scatters of
light. Cloud droplets and ice crystals though are
much larger than air molecules. Because of this they
scatter all the colors in equal amounts.
When white light strikes a cloud,
white light is scattered and reflected. This is why
clouds are white (with some shades of grey mixed in if the
cloud is thick). When you look up at a cloud you see
a white cloud (sunlight being scattered by cloud droplets)
surrounded by blue sky (sunlight being scattered by air
What about particles? Particulates are much bigger
than air molecules and a little bit smaller than cloud
droplets. They scatter light is the same way that
cloud droplets and ice crystals do. The scattered
light from particles is white.
Here's a pretty good illustration of sunlight being
scattered by particles. There must be a lot of dust
and particulates in the air in this photograph because you
can see the rays of sunlight shining through windows in
the dome of St. Peter's Basilica (Vatican City).
Note that the scattered light is white. (source
of this image) You can see something like
this outdoors when sunlight shines through breaks or holes
in clouds, the beams of light are called crepuscular
What if you particulates and air
together? What do you see now when you look at the
sky? It depends on how much particulate matter is in
the air. When the air is clean and doesn't contain
much particulate matter the sky is a deep blue.
You're seeing just sunlight being scattered by air
molecules. As the concentration of particulates
increases you mix in more and more white light. The
color of the sky can change from deep blue to a whitish
blue when the particulate concentration is high.
Because you have
learned a little bit about the scattering of light you can
understand why particulates affect visibility
In this first picture we start out with clean air. When
we look at a mountain we see the light that is reflected off the
soil and trees on the mountain (shown at left above). I've
colored this reflected light green and brown. When you look
at the mountain it's green and brown (right figure above).
Now we'll add some particles. When you look at the
mountain you see brown and green light plus some white light that
is coming from sunlight being scattered by the particles (the
white light is colored yellow in the figure at left). Some
white specks of light have been superimposed on the view of the
mountain at right.
Just a single picture now, what you would see when you look at
the mountain. More particles, more scattered light, and more
white light being mixed in with the brown and green reflected
light from the mountain.
Even more particles. Now the white light from scattering
from particles begins to dominate. Eventually it becomes
difficult to even make out the mountain because of all the
scattered light. Light from the mountain also runs into
particles on its way toward your eyes and gets redirected so that
you don't see it. Of course there was considerable artistic
license used in this illustration.
You might think that when the air is clean that visibility
might be unlimited. That isn't the case. Scattering of
sunlight by air molecules alone puts a limit on
visibility. The following figure tries to explain why this
As the mountain gets further away you start seeing increasing
amounts of blue light (sunlight scattered by the air in
between you and the mountain) being added to the brown and green
reflected light. This is because there is more
air between you and the mountain. The mountain
at medium range now appears brown, green, and
blue. As the mountain gets even further away
the amount of this blue light from the sky increases and the
green and brown light from the mountain weakens. The most
distant mountain in the picture above is now blue.
Eventually the mountain gets so far away that you only see blue
light from the sky and none of the light reflected by the
mountain itself. The mountain has faded from view.
Here's a photograph of the Blue Mountains in Australia (source
of this image)
The nearby mountain appears green and brown. You are
mostly seeing sunlight reflected off the mountain.
If you look closely I think you can see 5 mountain ranges in
this picture (1 is closest, 5 is the most distant). Notice
how they became fainter and fainter and lighter and lighter
blue. It becomes hard to distinguish mountain range 5 from
the blue color of the sky.
Clouds are normally white. Around sunrise and
sunset clouds often become yellow, orange, and red. The
reason for this is that the color of the sunlight that is
shining on and being reflected by the clouds changes.
When the sun is low in the sky the rays of sunlight
take a much longer path through the atmosphere and there is
more opportunity for light to be scattered (and absorbed).
At Point 1 in the figure below we assume the incoming
sunlight is white because it is a mixture of equal amounts of
all the colors. After this sunlight travels a short way
through the atmosphere some of the shorter wavelengths get
scattered and removed from the incoming beam of light.
The scattered light, Point 3, is
what you see when you look at the sky. Only a small
fraction of the light in the original beam is scattered, so
it's not as intense as the original beam and safe to look
at. It's also colored blue.
The unscattered light, at Point 2, is the original
mix of colors with a little bit of some of the shorter
wavelengths removed. This can change the color slightly
form white to a warmer shade of white (colored yellow in the
When the rays of sunlight take a longer path through the
atmosphere much more scattering can occur. With enough
scattering, almost all of the shorter wavelengths can be removed
from the original beam of sunlight. This turns the
unscattered light orange or red and is shown at Point 4 above.
Here's a pretty impressive multiple exposure showing the
changing color of the setting sun (source
of the image). Notice also that the unscattered
light becomes less intense as the sun gets closer and closer to
the horizon (even so you shouldn't look directly at the sun).
It isn't necessary to risk looking directly at the sun to observe
how its color changes as it sets. Instead you can just look
at the sunlight that is reflected by clouds near the
Because the sunlight that strikes the clouds at sunset
(or sunrise) is orange or red, the clouds themselves will
appear orange or red.
of the image above)