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Thinking of radio waves as light

J

John Smith

Jan 1, 1970
0
Radio waves, being on the em spectrum, is light. So, can I imagine being
able to see radio waves and imagine seeing that light shining through
windows? And does it shine through thin walls, at not at all? Many thanks.
 
J

John Larkin

Jan 1, 1970
0
Radio waves, being on the em spectrum, is light. So, can I imagine being
able to see radio waves and imagine seeing that light shining through
windows? And does it shine through thin walls, at not at all? Many thanks.

Radio wavelengths are long, so the view will be fuzzy. It will shine
through most walls very well, but metal (wires, pipes, even nails for
the shortest wavelengths) will reflect, and even insulators diffract
and reflect a little.

Given a small board full of patch antennas and a lot of DSP, it should
be feasible to make a short-range UWB (baseband impulse) imaging
radar, maybe packaged sort of like binoculars with little LCDs inside.
That would be a great consumer product and a hell of a stud finder.

John
 
D

Dana

Jan 1, 1970
0
John Larkin said:
Radio wavelengths are long,

Only low freq radio will have long wavelengths. From 800MHZ on up they get
pretty short.
 
J

John Smith

Jan 1, 1970
0
John Larkin said:
Radio wavelengths are long, so the view will be fuzzy. It will shine
through most walls very well, but metal (wires, pipes, even nails for
the shortest wavelengths) will reflect, and even insulators diffract
and reflect a little.

Given a small board full of patch antennas and a lot of DSP, it should
be feasible to make a short-range UWB (baseband impulse) imaging
radar, maybe packaged sort of like binoculars with little LCDs inside.
That would be a great consumer product and a hell of a stud finder.

John

That's intensely interesting. I'm trying to get a feel for why the radio
goes out when I stand in certain parts of the room, mostly near the large
window. I thought, well if radio wave are like light then maybe they are
shining through the windows and maybe I'm casting a shadow. I'm having
trouble seeing how this light can shine through walls merely because there
is more of a delay between one pulse of energy, a wave, and the next. I know
that from watching welders that high frequency light casts sharper shadows
at a further distance than regular light... but I'm not certain why that is,
either. Refracting less? But, why?

Also, if you wouldn't mind: What is a patch antenna? What is a DSP? What
is a "base band impulse"? I know, I'll look it up, but I want to see your
answers, too. Many thanks.
 
J

John Larkin

Jan 1, 1970
0
Only low freq radio will have long wavelengths. From 800MHZ on up they get
pretty short.

800 MHz is still 0.375 meters, not very good for, say, shooting a game
of pool. Even x-band radar is still 3 cm, damned fuzzy compared to
normal vision.

John
 
J

Jim Large

Jan 1, 1970
0
Dana said:
Only low freq radio will have long wavelengths.
From 800MHZ on up they get pretty short.

800Mhz... That's 37.5 cm. Shorter than some
wavelenghths, longer than others. In fact it's
around eight hundred times longer than the middle
of the AM broadcast band, and its around five
hundred THOUSAND times longer than the longest
visible wavelengths. Don't be surprised if it
behaves a little differently from visible light.

-- Jim L.
 
J

John Larkin

Jan 1, 1970
0
That's intensely interesting. I'm trying to get a feel for why the radio
goes out when I stand in certain parts of the room, mostly near the large
window.

What often happens is interferance. If the waves take multiple paths
from the transmitter to you, the paths can get 1/2 wave (etc) out of
phase and cancel. Move a few feet, and you can find a spot where they
reinforce. Just like optical interferance fringes.
I thought, well if radio wave are like light then maybe they are
shining through the windows and maybe I'm casting a shadow. I'm having
trouble seeing how this light can shine through walls merely because there
is more of a delay between one pulse of energy, a wave, and the next.

Light will shine through materials thinner than a wavelength, even
thin metal films. Ditto radio: the dimensions are just all much
bigger.

Your body is too small to cast much of a shadow at AM and FM
frequencies... the waves just flow around you, like a popsicle stick
poking up on the beach.
I know
that from watching welders that high frequency light casts sharper shadows
at a further distance than regular light... but I'm not certain why that is,
either. Refracting less? But, why?

That's true, but probably not actually visible. An arc welder is just
a very small, intense source so it makes nice shadows.
Also, if you wouldn't mind: What is a patch antenna? What is a DSP? What
is a "base band impulse"? I know, I'll look it up, but I want to see your
answers, too. Many thanks.

Oh, OK.

A patch antenna is a little, well, patch of copper on a
printed-circuit board, designed to be an antenna. Cheap.

DSP = Digital Signal Processing. You'd need a lot of number crunching
to turn a bunch of radar echoes into a 3D image.

A baseband impulse is just a very short, sharp pulse, not modulated
onto a carrier wave, just blasted into space. UWB (ultra wideband
radar) often uses impulses as the transmitted/received signal.

Read up on this stuff; it's cool.


John
 
J

Jim Large

Jan 1, 1970
0
John said:
I'm trying to get a feel for why the radio
goes out when I stand in certain parts of
the room, mostly near the large window. I
thought, well if radio wave are like light
then maybe they are shining through the
windows and maybe I'm casting a shadow.

Interference. Have you ever seen a laser
speckle pattern? The same thing happens on
a larger scale anywhere you have radio waves
bouncing around in an enclosed space (i.e.,
a room). There are strong spots where the
various reflection paths are in phase so
that they reinforce one another, and there
are weak spots where they are out of phase
and they cancel out.

You are not casting a shadow. You *WOULD*
cast a shadow (a faint one) if you were out
in space, and the transmitter were far away.
You don't cast a shadow in your room for the
same reason you don't cast a shadow on the
ground on a cloudy day. On a cloudy day,
the light is coming from all directions.
The radio waves in your room are coming from
all directions too because they're bouncing
off the walls of your house and all of your
neighbors' houses, nearby hills, etc.
I'm having trouble seeing how this light
can shine through walls merely because
there is more of a delay between one pulse
of energy, a wave, and the next.

The wave crests are not "pulses of energy."
I don't know what they are. I don't know
if anybody *REALLY* knows what they are.
All I know, and all that most garden-variety
physicists know is that whatever electro-
magnetic energy is, it seems to obey (ON
AVERAGE!) the same mathematical laws that
govern waves like on the surface of a pond.

My understanding of why certain wavelenghts
pass through some materials and not through
others is pretty weak, but I know that it
has to do with the fact that electrons in
the material are confined to certain "energy
levels," and they interact with photons of
different wavelengths (i.e., photons of
different engery) only when the difference
between two allowable energy levels for the
electron is compatible with the photon
energy.

What is "allowable" and what is not
allowable depends on the the atoms in the
material and on the way they bond to one
another, and it's so far over my head it
isn't funny.
I know that from watching welders that
high frequency light casts sharper shadows
at a further distance than regular light...
but I'm not certain why that is

Ah, shadows again.

That's a simple geometry problem.
A welder's arc is a very compact, very bright
source of light. A compact source casts
sharper shadows than a larger source, and the
fact that the source is very bright makes the
shadow very noticible.

-- Jim L.
 
J

John G

Jan 1, 1970
0
The AM Broadcast band is about 500 to 1600 kilohertz and that is 600 metres
to around 185 metres which is quite a bit longer than 37.5 centimetres not
shorter.
But it is mny times longer then visible light.
 
J

Jim Large

Jan 1, 1970
0
John said:
The AM Broadcast band is about 500 to 1600 kilohertz
and that is 600 metres to around 185 metres which is
quite a bit longer than 37.5 centimetres not
shorter. But it is mny times longer then visible light.

Ooops! I meant to say "shorter," really!

-- Jim L.
 
R

Rich Grise

Jan 1, 1970
0
It would easily be visible, if you have eyeballs the size of a high-rise
apartment building. ;-)

Cheers!
Rich
 
W

Watson A.Name - Watt Sun

Jan 1, 1970
0
Radio waves, being on the em spectrum, is light. So, can I imagine being

Radio waves are electromagnetic radiation as are light waves. But
each is in a completely different part of the spectrum, so saying
"radio waves .. is light" (sic) is not true.
able to see radio waves and imagine seeing that light shining through
windows? And does it shine through thin walls, at not at all? Many thanks.

Radio waves can penetrate walls like light penetrates glass. But not
all wavelengths of light can penetrate, as not all wavelengths of
radio waves can penetrate walls.

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T

Tom Del Rosso

Jan 1, 1970
0
In John G typed:
The AM Broadcast band is about 500 to 1600 kilohertz and that is 600
metres to around 185 metres which is quite a bit longer than 37.5
centimetres not shorter.
But it is mny times longer then visible light.

All this talk about objects thinner than a wave being transparent makes
me wonder about antennas. So then why does an antenna not have to be
more than a small fraction of a wavelength?

And what about the principle of shielding as in the door of a microwave
oven? The rule there seems to be that the screen is opaque because the
holes are smaller than a wavelength, but that measurement is
perpendicular to the wave. Why?
 
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