Scattering
We see things light up every day because of
scattering. We see the effects of scattering
from the time we wake up in the morning
until we shut off the lights before we go to
sleep at night.
Our sky is blue because of the
scattering of light that occurs in the
atmosphere. Earth’s atmosphere is mostly made up of
Nitrogen. The colors that we see in the sky throughout the
day depend on the size and composition of the particles in
the sky. This scattering process is known as Raleigh
Scattering.
Have you ever driven through a patch of fog at night?
Before you drive through the patch of fog, your headlights
can barely be seen. As soon as you progress into an area of
fog though, the fog is lit up and your visibility decreases.
Because of the fog droplets you can no longer see very far
in front of your car. The cone shaped area that is lit up the
most is directly in front of your headlights. This occurs
because the water droplets in the air scatter the light.
Why do clouds and crepuscular rays appear white?
For the same reason that the fog droplets are white when
you shine your high beams at them. When the particles are
large (water droplet size) all of the light is scattered
equally. We see white light scattered when there are water
or ice crystals in the air. Each wavelength of visible light
scatters the same amount when the light hits the water
droplets.
Raleigh Scattering
Strange things happen as light rays enter our
atmosphere from the sun.
As the light enters the earth’s atmosphere it hits many
small air molecules. In our atmosphere (made up of 78%
Nitrogen, 21% Oxygen and 1% Argon) most of the
scattering particles are very small. These air molecules are
bombarded with electromagnetic waves (see document)
from the sun and begin to vibrate. While vibrating these
particles reemit the electromagnetic waves. For example if
we have a vibrating paddle in a pool, it will create water
waves. Like the vibrating paddle, the particles create
waves that pass through space.
"Image/Text/Data from the University of Illinois
WW2010 Project."
Not all particles in the atmosphere reemit the light the
same way. Small particles reemit light equally in all
directions (see Figure 2-1)(A) and larger particles focus the
light mostly in the direction opposite that of the sun (D).
Figure 2-1 “Rainbows, Halos, and Glories”
When the light reaches your eye from the scattering
particles, you see light coming from all directions. The
scattered light is emitted by the particles in our atmosphere.
Without these particles our sky would be dark. Outer space
is dark because there are no particles to scatter the sun’s
light.
Effects of Particle Sizes
Small particles (the size of a Nitrogen Molecule)
scatter light with very small wavelengths the best. In our
mostly Nitrogen filled atmosphere, blue light is scattered
the most. This is why our sky is blue.
Have you ever seen a brilliant red sunset? After all of
the colors have scattered out of the white light, we see the
oranges and reds. Where on the horizon are the red colors
found? The reds are found close to the horizon because the
sunlight must pass through many particles before we reach
the point that red is scattered out. The blues, greens,
yellows and oranges have to scatter out as the sun passes
through the largest distance through our atmosphere.