Re: Does the earth's atmosphere act like a gas lens?

Greetings Harry:

References:
1.. Handbook of Chemistry And Physics,
CRC - McGraw-Hill Press, any edition

2. Reference Data for Advanced Space Communication and Tracking Systems,
Volume 4, NASA Goddard Space Flight Center, Report 5-9637,
Prepared by the Hughes Aircraft Company, October 1969

3. Transmission Systems for Communications, Fourth
Edition, Members of the Technical Staff, Bell Telephone
Laboratories, Western Electric Company Technical
Publications, December 1971.

Yours are very interesting questions and the answers may surprise you!.

The optical index of refraction of a material (usually called the index) is a measure
of the speed of light in the material. Vacuum has an index value of one. The earth's
atmosphere has a index slightly greater than one and varies with altitude. A glass
with an index of 2 slows the speed of light by one half relative to the speed of light
in a vacuum. A material with an index of 3 slows the speed of light to 1/3 that in a
vacuum and so on. Most common glass lenses have an index between 1.5 and 2 and the
index is the same throughout the material. The focusing properties of these lenses
are determined by the curvature of the glass surfaces.

Recently lens formed from glass with a tapered index of refraction have been developed
and are called a graded index lenses. In recent years graded index lenses have been
developed for fiber optic applications. A typical graded index lens is in the form of
a cylinder yet it exhibits, depending on the grading of the index, the refracting
characteristics of convex or concave lenses. Graded index spherical lenses have also
been developed for use at microwave frequencies and are called Luneberg Lenses.
, named after the inventor. Because of their spherical symmetry, the Luneberg Lens
has a 360 degree field of view in all spatial directions. A wave front entering the
lens from any direction in space is focused near
the back surface of the lens directly opposite from the incident wave. This type of
lens is ideal for communicating with several satellites by simultaneously forming
several beams in space without having to steer the spherical lens. Only the feed
points in the focal sphere must be moved physically or electronically.

The earth's atmosphere acts as a graded index lens because the index varies from
1.000 at the beginning of space to about 1.00029 near the earth's surface. The imaging,
and distorting properties of mirages are caused by lens like bubbles or layers of hot
surface air with greater index surrounded by cooler air with a lower index. Because
this interface is a graded index, the mirage images can be distorted or even inverted.

When light passes through the earth's atmosphere the particles and gas molecules in
the atmosphere absorb and reflect the light. This reflection is in many directions and
is called scattering. Scattering is wavelength dependent because the size of the
particles determine what band of wavelengths are scattered. This is why the sky is
blue overhead. This absorption and scattering severely reduces the amount of light
propagation in all directions and destroys any lensing effect that the graded index
of the atmosphere might create. For example, reference 2 has data on the amount of light
scattered in the atmosphere, after sunset, on a moonless night when looking vertically.
We call this twilight! The earth rotates 15 degrees per hour. By 40 minutes after
sunset the power of the light scattered vertically is reduced to 0.5% of the light
scattered just after sunset. By 60 minutes after sunset (15 degrees around the earth)
the level of light power scattered from the vertical is 0.0005% of that at sunset.
At this level the light from the stars sets the light level observed vertically.
So the answer to your question is that because of scattering and absorption of
light in the atmosphere no lensing effect is observed except for mirages.

Microwave and radio waves are much longer than optical wavelengths and the atmosphere
is very transparent at these frequencies. When we aim a microwave beam at the horizon
the beam bends over the horizon and travels 30% farther than the visual line of sight.
This refraction is cased by the graded index of the earth's atmosphere. During certain
weather conditions where the atmosphere has an inversion layer, microwave beams have
been detected hundreds of miles beyond the horizon.

At radio frequencies lower that about 100 MHz (100 million cycles per second) radio
signals are reflected off layers in the ionosphere and can travel halfway around the
earth. These signals occasionally arrive at a receiver after traveling both ways around
the earth between the transmitter and receiver. At the distances where these waves
interfere and are in phase the signal can be considered focused at the receiver.
Thus at radio and microwave frequencies the answer to your question would be
yes, the earth's atmosphere does act like a spherical lens with a graded index.

I have discussed these phenomena in detail in the Mad Science Archives at the
following addresses.

Re: How is it possible to bounce a signal off of the troposhere?

ht tp://www.madsci.org/posts/archives/feb2002/1013615817.Ph.r.html

Re: RF 2.4Ghz Signals and Weather Conditions

htt p://www.madsci.org/posts/archives/dec2000/975943217.Ph.r.html

Best regards, Your Mad Scientist
Adrian Popa