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Light Interference
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Question:
How is that given the myriad of electromagnetic waves
generated by radio, TV, mobile phone, remote control devices, etc. etc.
these wave forms appear to retain their integrity and do not interfere with
one another ?
Replies:
Well, they DO interfere with each other, but that doesn't make them go
away. You can think of it like waves in the ocean. Many waves with vary
different periods and amplitudes combine, without any of them disappearing.
The large swells roll onto the beach, and atop the swells are myriad
smaller waves going every which direction. If you swim out beyond the
breakers and throw a rock into the water, ripples will radiate away from
the impact just as they would in a glassy pond. They do this on top of all
the other waves, and their effect is added to the effects of all the other
waves, but they still are there.
The circuitry that decodes the signals from radio waves is sensitive to
only a small range of wavelengths, and anything outside that range isn't
even detected. So, if your circuit is tuned to the "ripple" frequency, it
won't even know that the "sea swells" are there. Of course, that's only
part of the problem. If a hundred people are throwing pebbles into a
glassy pond, it is very difficult to discern the ripples from any given
rock. That's the challenge of maintaining a cellular phone network. I
don't know exactly how that is done, though I know that it involves a
number of clever tricks, such as assigning slightly different frequencies
to different devices depending on what other frequencies are in use at the
time and special identifier signals so that the network knows which signal
comes from which phone.
Richard Barrans Jr., Ph.D.
Hello,
They actually do. That is why when you are on your cordless phone, you can
may hear your neighbors! To avoid that, you sometimes can switch channels.
Anyway, in the US, FCC, the Federal Communication Commission, regulates the
frequencies and strengths of the broadcasting centers as well as other
sources of electromagnetic emission form devices such as motors, microwave
ovens, etc. (Look on the label on the back of these and you see FCC
compliance).
If broadcasts are made at different frequencies (channels) OR are
adequately distant from one another, then your receiver (radio or cordless
phone) which is tuned to a particular frequency will pick only the one you
are tuning to.
Do these waves collide in the air, sure they do. Do they get distorted a
little, yes.
Can we tell just by listening in? Not usually, and, in addition, all kinds
of "filters" are used to take out these interferences.
Interference effects also depend on the kind of broadcast, frequency
modulated (FM) and amplitude modulated (AM). They are affected differently.
Finally, if the interfering source is both broad and strong enough, you can
get the wave you are tuning to distorted enough to overcome your filtering.
This This is the limit of my knowledge. May I suggest that you look at FCC
site (www.fcc.org) and links therein for more information.
Dr. Ali Khounsary
The quick answer is they actually due interfere with each other.
A prime example is solar flares interacting with radio and satellite
signals on the earth.
Dr. Myron
Gracious, of course they interfere with each other. Perhaps the
conceptual block here is that you assume "interfere" means "destroy
the information content of", that is, you are wondering why the
presence of the local Top 40 FM radio station antenna in your
neighborhood does not hopelessly muddle your reception of TV signals,
or mobile phone signals, etc.
When two radio signals are in the same region of space, they
"interfere" in the same way as two ocean waves that occupy the same
area of ocean interfere: the net amplitude of the electric field (or
ocean height) is the sum of the amplitudes of the two waves. Granted,
this produces a much more complicated-looking electric field
distribution (or sea surface distribution), but no information has
been lost. You can readily disentangle one wave from the other with
your receiver. The first thing your receiver does is represent the
complicated looking electric field by the sum of a large number of
pure AC sinusoids of various frequencies. Fourier's Theorem says this
can always be done. It then throws away all of these sinusoids that
are at uninteresting frequencies, for example those below 88 MHz and
above 105 MHz if you are listening to FM radio. Actually, you select,
by means of your tuning dial, an even sharper filter that throws away
all waves at frequencies more than a Mhz or so away from a particular
target frequency, KROQ 102.6 FM, or whatever. It is at this point,
you see, that your receiver has thrown away all the "interference"
form TV, mobile phone, etc. transmitters nearby.
Dr. C Grayce
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Update: June 2012
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