Transcript Scattering and Polarization of Light

Scattering and Polarization of
Light
Why is the sky blue?
Why is the setting sun red?
Scattering of Light
“Scattering” of light is what we call the
phenomenon in which light waves interact
with particles (molecules) and are removed
from an incident light beam.
Scattering of Light
How big is a molecule ??
A good estimate is to say that most
molecules are smaller than about
1 nm in size (0.000000001 m)
Scattering of Light
Molecules in air scatter light, but we usually
see this only weakly.
We can enhance the scattering by producing
more scatterers …
for example, by putting chalk dust in
the path of a laser beam.
Scattering of Light
Question: if the amount of scattering depends on
The number of scatterers, why do we not see
Much scattering when a laser passes through glass?
Answer: We return to the idea of interference.
For a relatively dense medium, there is a good
chance that for each molecule scattering light,
another one is one-half wavelength away.
Scattering of Light
For light scattered sideways from a beam,
this results in destructive interference.
In the forward direction, the interference
is constructive, so the result is that the
beam continues to propagate.
Rayleigh Scattering
Scattering of white light from very small
objects depends strongly on the wavelength
of the light - short wavelengths are
scattered most easily
*smaller than the wavelength
Rayleigh Scattering
Light passing
through more
of the atmosphere
is scattered more.
The sun at noon
appears pale
yellow, and at
sunset it is red.
Rayleigh Scattering
The sky appears blue, for the same reason;
as we look up in the sky, we see light that
has been scattered by molecules.
More blue light is scattered than green or red,
although some of these colors are present as
well. The result: we see unsaturated blue.
(Red light keeps going over our heads and
out the other side of the atmosphere.
Scattering
An experiment:
Put a few drops of milk into a container
of water. Shine a flashlight through the
water. The transmitted light looks orange
or red (looking back into the flashlight),
while the light scattered out the sides is
blue or gray.
Scattering
Another demonstration of scattering can be
done during your cigarette break. Why is
the smoke bluish in color (especially when
viewed against a dark background) ?
Mie Scattering - Large Particles
Now consider the cigarette smoke that
is exhaled. Does it look blue?
Exhaled smoke tends to look pretty much
white (or gray).
Water condenses on the particles making
them much larger - scattering is
independent of the wavelength.
Mie Scattering
Why are clouds white (cumulus clouds)?
Lots of large water droplets scatter light
of all wavelengths equally. Light is
scattered multiple time in traversing
the cloud.
Scattering
Why is the sky black when seen from
outer space?
There are no particles to speak of, so
there is no scattered light; we thus
see a black sky.
Polarization
x
An “x-polarized” field
z
y
(Side view of wave)
Polarization
y
y
x
y
x
“Head-on” view of polarization
x
Polarization by Scattering
Rayleigh scattering in addition to being
wavelength dependent is also polarization
dependent (leads to polarized light).
Polarization by Scattering
y
x
Scattering molecule
z
Electric field causes charge to oscillate
Polarization by Scattering
y
x
Scattering molecule
y-polarized wave
z
x-polarized wave