Talking Points for
“Information In Waves” Presentation
At
this point, students should have a basic understanding of wave terminology and
the electromagnetic spectrum. This presentation
is designed to help them connect these basic concepts to the use of
electromagnetic waves.
This
presentation will describe how each of these ways of encoding information is
related to a fundamental attribute of the source/receiver combination:
All
wireless communication is done via the transmission of electromagnetic
radiation. The information itself rides
along the wave in one of the four attributes of the waves. The transmitter and receiver have an agreed
upon “key,” or method, of encoding or decoding the information.
Scientists
use these same attributes of waves, coupled with their understanding of the
natural phenomena that create them, to “decode” the waves from space. Scientists can infer how and where they were
created, or what the waves have passed through on their way to Earth.
The brightness of an
object is basically a count of how many photons are entering your eye from the
source. Many photons traveling together
no longer seem like individual packets to our detectors, and so their number is
represented simply by the amplitude of the wave from that source.
Even
simpler than AM radio are codes such as Morse code, in which the amplitude is
either full strength or zero.
E = hf give us the
relation between the quantum explanation of the photon’s energy and the wave’s
frequency. Note that the absorbed wave
must also raise the energy level of the electron by the same amount as
contained in the photon.
Color is actually an overlay of only three
different receptors in your eye that are keyed to high, middle, and low
energies. The relative weighting between
these energies gives us the entire spectrum of color as we experience them.
The
Doppler shift can tell us about the relative motion of interstellar
objects. Notice the spectral absorption
line in the middle of the graphic. An
absorption line occurs when the waves travel through a specific absorbing
medium (frequently hydrogen). The absorbing medium will then take out only
those energies that correspond to their precise energy gaps. By observing these missing pieces, we can
determine the content of the interstellar gas.
The amount by which the lines are shifted in frequency will give us the
speed of the source.
Note
that the terminology “blue shifted” for objects heading straight for us, and
“red shifted” for those headed away, are illustrated in the graphic.
The fact that FM radio
does not carry further at night is not related to the fact that the
information is in the frequency, but rather the fact that FM exists at much
higher frequencies (MHz rather than KHz) than AM. Therefore, FM radio waves penetrate the
atmosphere and do not bounce back over the horizon.
The
online demo can be use to demonstrate much more than interference via the phase
shift. The bottom left diagram is the
sum of the upper two waves. To
demonstrate phase interference, do the following:
1.
Place
the red and grey balls on top of each other in the lower right hand panel. The two waves should have the same
wavelength and amplitude.
2.
Now
grab the upper right wave anywhere other than on the red ball. Pull slowly to the right or left, all the
while watching the superposition of the two waves go through cycles of
constructive and destructive interference.
Note
that the background graphic uses the standard symbol phi for phase and shows
that phi is equal to angular frequency (omega) times time.
If
an interstellar wave travels though some plasma or gas on its way to Earth, it
will be slowed down and thus be slightly phase shifted.
VLBI
(Very Long Baseline Interferometry) is a way to create an effectively huge
telescope from various independent telescopes located a known distance
apart. It is essential that the various
telescopes must agree on the time when they received their signals; therefore,
they have synchronized atomic clocks.
A well-known form of RFID (Radio Frequency
Identification) is the EZPass toll system.
RFID is also gaining popularity in retail and for tracking purposes in
general. Pictured here are three common
types of RFID in which an ID code is contained in the phase variation that the
tag sends to the reader.
A passive system is one in which the tag has no
power source of its own. The tag uses
the power of the incoming signal to activate its circuit and bounce back the
same signal, phase modified.
A semi-passive system is one in which the tag has
power for its circuitry, but does not broadcast the radio signal. The tag bounces back the reader’s signal.
An active system is one in which the tag has power
for circuitry and radio. The tag uses
its own power to broadcast its ID to the reader.
Depending
on the orientation of the reflecting electrons, the scattered signal will be
preferentially polarized (as will be seen in a later slide). Note that some sources of radiation (such as
synchrotron radiation from high energy electrons) are naturally polarized.
Although
our eyes are not sensitive to polarization, oftentimes it is the scattered and
reflected light that hurts our eyes or obscures the light we want to see. Note that scattered and reflected light is
more polarized. By eliminating all
polarized light in a certain direction, the scattered light will be almost
completely eliminated whereas the normal, unpolarized light will only be mildly
affected.
Just
because human eyes cannot differentiate between polarizations does not mean
other animals are so limited. The
pictures are from a researcher who is trying to incorporate real time polarized
light visualization techniques to supplement “normal” visual information.
Faraday
rotation is very important when determining how much magnetized plasma the waves
have gone through.
In
industry, some testing devices rely on the natural optical activity of some
molecules for testing purposes. (Note
that this is not Faraday rotation because it does not rely on magnetic fields.)
The
Faraday effect is seen when polarized light enters a magnetic field in the same
direction at the light beam. The plane
of polarization is rotated some amount depending on the plasma density, the
magnetic field, and the wavelength.
For
more information, see http://www.answers.com/topic/faraday-effect.
The
two signals do not interfere. In
general, if a linear antenna is not lined up parallel with the polarization,
the signal will not be seen. A linear
antenna initially sends out a polarized signal, but as the waves are reflected
and refracted, they will lose their original orientation