|MadSci Network: Physics|
Reference: H. C. van de Hulst, "Light Scattering by Small Particles",
One of the marvelous features of laser light is that it can be formed
into a very narrow beam and we can use small amounts of power to
communicate over great distances, if the atmosphere is clear enough.
When a light beam passes through a vacuum, through space or through
very clear air it is not possible to see the beam with human eyes
unless you are inside of the beam looking toward the source of the
light. This is why we can bounce laser light off of the moon and
measure the distance to the moon with an accuracy to a few centimeters
(inches). However, the earth's atmosphere, particularly the first 3000
meters (yards) in altitude, is filled with particles of dust, haze,
pollutants, water vapor and other molecules that absorb and reflect
The reflection of light out of a light beam by particles is technically
called multiple scattering as the light energy bounces from
particle to particle as it leaves the beam. This sideways scattering
of light out of the beam enables us to see the profile of a beam from
outside of it if the scattered power is great enough. The intensity of
the beam in the atmosphere can rapidly decrease with distance, particularly
in the first few 1000 meters as a great amount of the energy is scattered
out of the beam by the particles in the atmosphere.
When the particles in the air are spherical and large compared to the
wavelength of light, such as water vapor in a cloud or fog, the beam is
absorbed and scattered in all directions (omni directional) and bounces
back and forth through the particles lighting the entire cloud. This is
what happens when sunlight, lightening or a search light enters a cloud.
(NOTE: the wavelengths in visible light range from about 600 to 400 nanometers
( a nanometer is 0.000,000,001 meter).
I have two laser pointers, one is red operating at a wavelength of
623 nanometers and the other is green operating at 520 nanometers. Both
lasers have an output power of one milliwatt (0.001 watt). When I shine
the two lasers into the night time sky, I can see the green beam like a
search light going into the sky but I cannot see the red beam. Why is
this happening when the lasers have equal output power? First, the shorter
wavelength of the green light reflects more power sideways out of the
beam from because the diameter of the particles in the atmosphere are greater
in numbers of wavelengths to the wavelength
of the green light than to the red light. However, second and more important,
the human eye is more than ten times more sensitive to green light than to
Back in the 1968 I participated in an experiment to transmit laser light
to a Surveyor VII Spacecraft resting on the lunar surface. The picture
from a television camera on the spacecraft was then transmitted by microwave
back to the earth. We used a one watt argon ion laser that produced green
light at a wavelength of 514 nanometers and a 30 inch telescope operated
by Caltech located on Table Mountain, near Wrightwood, California to form
a beam that was about one mile in diameter on the lunar surface. Even in the
clear mountain air we could see the beam rising like a green search light
toward the moon. Because of the clear atmosphere the beams from Table
Mountain and from the Kitt Peak National Observatory near Tucson, Arizona
were observed by the spacecraft as bright lights on the night side of the
earth, while beams from three telescopes in the eastern USA were not seen
because of atmospheric loss of light power from clouding. This 384,000 km
(240,000 miles) distance was a record distance for transmitting laser
light for about 30 years until NASA recently communicated with a spacecraft
more than a million miles from earth with laser light.
When I perform demonstrations of laser beams I use a spray can of air
freshener to produce particles to scatter light from the beam so that
the beam can be observed. Some laser light shows will generate a small
amount of smoke to show the beams. To observe laser beams in water I
add a small amount of milk and the milk particles scatter enough light
out of the beam to observe it. Sea water greatly absorbs and scatters laser
light; however, green light does pass through sea water with less loss
than red, orange, yellow, blue and violet wavelengths suffer. There have been
a number of experiments to communicate with submerged submarines using
green light; however, the results of these experiments are mostly classified.
Powerful laser radars called LIDARS (LIght Detection And Ranging)are
now being used to detect and measure clear air turbulence in front of
high flying jet aircraft by monitoring the back scattered laser energy
from the small numbers of particles in the upper atmosphere. These LIDARS
transmit pulses of light that have many thousands of watts of peak power
to be able to detect the small numbers of particles.
LIDARS have recently made the first three dimensional pictures of the
ice caps on the planet Mars.
http://ltpwww.gsfc. nasa.gov/tharsis/agu_f98.html http://ltpwww.gsfc. nasa.gov/tharsis/agu_f98.html
Evedently the Martian atmosphere is clear enough for laser energy to
reach the surface and be reflected back to the orbiting spacecraft
In space the average particle size and density is so small that we cannot
detect it; however, the gas clouds in deep space will absorb and scatter
laser light. This has been observed by observing laser light generated by
clouds of gas near stars that form natural lasers in deep space.
Best regards, Your Mad Scientist
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