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Author Topic: How do radio waves travel?  (Read 7537 times)

Johann Mahne

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How do radio waves travel?
« on: 02/08/2010 17:30:02 »
Johann Mahne  asked the Naked Scientists:
   
Hi Chris,

I find radio waves fascinating.

They can penetrate any type of insulating material, for example, concrete walls that are dry.

How does this happen?

We are told that radio waves have two parts, one showing an electric potential and one a magnetic component.

How do these fields pass through the molecules of the wall and get reconstructed on the other side? I guess they do the same thing through air molecules, but the molecules are spaced much further apart....

Cheers
Johann

What do you think?
« Last Edit: 02/08/2010 17:30:02 by _system »


 

Offline princecheck13

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How do radio waves travel?
« Reply #1 on: 02/08/2010 14:31:00 »
Path of least resistance to earth discoveries for the Lightning. If the rubber is falling blocks will be surrounded by air and then will hit the air as no less than rubber in terms of electrical resistance is ohms.
 

Offline graham.d

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How do radio waves travel?
« Reply #2 on: 05/08/2010 13:24:23 »
princecheck13, I have no idea what you are saying.

Johann, the absorption, reflection or transparency of a material depends on how the material reacts to the changing electric field associated with the electromagnetic radiation (discounting magnetic effects). For metals, electrons are free to move between atoms; if they can move fast enough they will be induced to follow the field and can be thought to re-radiate at the same frequency as the incident wave producing a reflection. For non-metals the electrons are "attached" to the nuclei of the atoms. They react to the changing field in ways that depend on their natural resonant frequencies. The wave can just end up heating the material very slightly whilst reducing the energy of the wave trying to pass through, and if the material is thick enough it will absorb all the energy. A wave of a different frequency may be absorbed by a particular resonance and then selectively re-emit this frequency (this is why materials have specific colours). If there is no resonance to the frequency, the wave will pass through. At very low frequencies there are unlikely to be resonances within the materials so radio waves tend to pass through such materials. Metal does not depend on the resonance mechanism so metal will reflect the low frequencies too.
 

Offline graham.d

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How do radio waves travel?
« Reply #3 on: 05/08/2010 13:29:57 »
I have just found this:

http://hyperphysics.phy-astr.gsu.edu/hbase/mod4.html

which is probably clearer than my ramble.
 

Johann Mahne

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How do radio waves travel?
« Reply #4 on: 06/08/2010 08:53:25 »
  Are you saying that the electrons in an insulator resonate with the wave components and in so doing transport the wave similar to a wave transporting the energy of a stone?
They would have to resonate with both components of the wave.Incredible if this really happens
 
 

Offline graham.d

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How do radio waves travel?
« Reply #5 on: 06/08/2010 10:25:03 »
If the electrons in an insulator resonate with the incoming em wave, they will reflect the resonant frequencies which gives rise to materials having a specific colour. Away from the resonance frequencies the electrons will vibrate with the frequency on the incident em radiation, but reaction with the surrounding material results in losses (it just causes the material to heat) and the wave energy disperses as it goes into the material. If there are no modes of vibration available the material will be transparent. There is no simple analogue to this outside quantum mechanics I think.

I don't think the magnetic component of the wave has any significant effect in causing the electrons to vibrate, although it clearly is key to the wave propagation.
 

Johann Mahne

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How do radio waves travel?
« Reply #6 on: 06/08/2010 13:06:11 »
 If the molecules did resonate away from the frequency of the wave, then as you say there would be losses, which would mean a reduction in amplitude of the wave. But they seem to manage to resonate at the wave frequencies.
  I don't understand how waves can pass through all insulators,with no lossses.There would be heavier molecules such as in a wall and lighter ones such as in air.Also their density would vary. So insulators should actually cause losses in waves.
  A radio wave propogating in space should have less losses than one through air?
 

Johann Mahne

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How do radio waves travel?
« Reply #7 on: 06/08/2010 13:17:11 »
Or are you saying that raio waves would penetrate depending on their resonant frequencies?
So wood and glass would tend to favour different waves, but reject others.
  This could be a good theory, but the problem is cell phone transmissions are in Ghz and radios are in Khz to mhz. So with this wide range , it's really hard to see how resonant frequencies of materials can be that broad.
 

Offline graham.d

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How do radio waves travel?
« Reply #8 on: 06/08/2010 13:29:01 »
You are quite right. There are always losses, but these may be quite small. Radio waves do not propagate through air as well as through a vacuum, for example, and the attenuation through a concrete wall can be very significant.
 

Johann Mahne

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How do radio waves travel?
« Reply #9 on: 06/08/2010 13:46:20 »
Ok so what is the conclusion?Can we need to think of walls and doors as diaphragms that can vibrate.They accept waves and pass them on? Strange..but true.
 
  What I'm still not clear on, is that magnetic waves such as from electrical equipment pass though insulators. Therefore your statement that the magnetic part of the wave is not important in the  transmission cannot be true?


  The really old question, how do waves propogate without a medium?
No doors or walls or water.

 We are told that light waves are particle and wave in nature, but radio waves are not.
 So I trying to visualize  a light wave progating as a string unwinding on a vibrating drum.The drum is the energy source that is vibrating and unwinding and converting to light, at the same time losing mass.I'm not sure if that's a good anology?Do you have a better one?
   
 

Offline graham.d

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How do radio waves travel?
« Reply #10 on: 06/08/2010 14:43:34 »
I did not explain all the mechanisms for losses. Walls and doors are insulators usually (not metals) but, to get more technical, they do have a relative permittivity which is greater than 1. They will react to the incident electric field and polarise to some extent. They also have the magnetic equivalent to permittivity (permeability) but for most common materials this will be close to 1. Losses from low frequencies (like radio waves) result from the inability of the medium to change the polarisation of the induced dipoles in response to the incident electric field. This gets worse as the frequency increases.

Magnetic waves can pass through insulators and, to some extent, many metals although with some attenuation. I did not mean to imply that the magnetic component was unimportent but just that, because it is often largely unaffected, calculations of the attenuation of an em wave passing through a material need only be concerned with the materials electrical properties.

All em waves can be considered as wave-particles. It is just that our more common observations would lead us to consider that very short wavelength em waves (such as gamma rays) behave more as a particle and long wave em radiation such as radio waves behave more like a wave. In the right circumstances they both can behave as their alter-ego though.

How waves propagate without a medium is not easy to answer and depends on how we thing the world (universe) is constructed. In em field theory a change in an electric field causes a change in a magnetic field which causes a change in an electric field etc etc. The fields exist in free space and require no medium. This is a big subject and ultimately it is based on what we observe to be the case. We can work out equations that describe and predict what happens and then try to fit a visual model that we can use to aid our thought processes. There is no reason why the universe should be conveniently analysable by the human mind though, and most aspects of physics, when gone into in depth, stretch this process to the limit. Ultimately the answer to "why" is because it is so!!
 

Johann Mahne

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How do radio waves travel?
« Reply #11 on: 06/08/2010 15:34:01 »
 Thanks for the explanation.
 When you say permittivity, I guess you mean the ability of a material to polarize, or do you mean its electrical properties?
  I did not realize that radio waves are also particle in nature.That means they have three components that must be passed thru insulators??.Or do the particles get reconstructed on leaving the material.
Can't really wrap my head around that one.
 Is the analogy that the materials are vibrating microscopically and that they behave as buffers
that accept and transmit waves correct or not?The molecules or electrons need to resonate along the same axis as the wave,surely?
 

Johann Mahne

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How do radio waves travel?
« Reply #12 on: 06/08/2010 16:01:23 »
 What I'm really saying is that the particle nature of the wave cannot possibly exist inside the wall only outside it.Big statement but I'll stick to it.
..Just been interrupted by a bubu shrike outside my window waiting for his cheese, now where was I?...
 This is deviating from the topic, but I have another question..
  What I saw last night on a school science tv was that the energy of a wave is only related to it's wavelength and nothing else. What about a light wave's amplitude? Radio waves must have their energy related to their amplitude as well, is that right?
 

Offline graham.d

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How do radio waves travel?
« Reply #13 on: 06/08/2010 16:22:09 »
Permittivity is an electrical property and it is related to its ability to polarise in an electric field.

Wave-particle duality is a difficult concept. Sometime em radiation behaves as a wave - it reflects, refracts, produces interference patterns etc - and sometimes it behaves like a particle - when it ionises an atom, counted by a photodetector etc. The particle (photon) is what has a discrete energy equal to (h is Plancks constant and ν is the frequency. A wave is made up of many such quanta which may be in phase, as in a coherent laser beam, or not as in a normal light source. The amplitude depends on the number of photons. 
 

Johann Mahne

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How do radio waves travel?
« Reply #14 on: 06/08/2010 17:00:49 »
  .Ok I'll give up on the duality,and the question if the particle component can exist inside the wall.

  Referring to the energy of a wave.If the amplitude depends on the number of photons, then over an interval of time more photons will hit the target.So how then can the energy only depend on the frequency?Unless waves of higher amplitude always have higher frequency. So maybe red light has less of an amplitude than ultraviolet.
  I'm also thinking that if you are looking at a star really far away it's light is dimmer, then wavefront must be less dense than the wave front of a star that is nearer.The wavefront is perpindicular in both cases. Hence the wavefront must have some energy related to photon density, and maybe this will reduce it's amplitude?So then my asumption of amplitude related to frequency must be wrong, because a star's colour does not change with distance.
 
 

Offline graham.d

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How do radio waves travel?
« Reply #15 on: 06/08/2010 17:35:48 »
Amplitude is not related to frequency, only to the number of photons. When looking at a distant star a telescope has to track the star to compensate for the earth's rotation and to get enough light intensity to record the image. Very distant objects can be so dim that the detector may only receive an average of (say) 1 photon per second. Each photon will have a particular wavelength and therefore a defined energy and, hopefully, enough to trigger a detection.
 

Johann Mahne

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How do radio waves travel?
« Reply #16 on: 06/08/2010 17:47:36 »
Right, accepted.
So then as you stated E = hV  or hF .
So then if two waves are of identical frequencies but of different amplitudes, then how can their energy be the same, and how can the energy only depend on F? 
 

Offline graham.d

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« Reply #17 on: 06/08/2010 18:16:04 »
Because one comprises more photons than the other. The photon is the quantum - everything has to be a multiple of this. If you wish to consider a light source; if you turn up the brightness (amplitude) it only goes up in discrete steps as the density of quanta is increased.
 

Johann Mahne

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How do radio waves travel?
« Reply #18 on: 06/08/2010 19:12:05 »
Sorry I don't understand.
 A wave with a higher frequency would contain more photons per unit length, and so also per unit time as the speed of light is fixed. I agree.
You stated that the number of photons(quanta) per unit wave length also depends on the amplitude, which seems to make sense to me.
  if the photons are of fixed length there would be more in a bigger wave and also more in a wave that is compressed.
 If an observer saw photons going past him, he would count more in a bigger wave and shorter wave
over 1 second and 1 meter.
So how can the energy equal only the frequency?

Should be e=hxFxAmp or something?

  The energy of a  radio wave should also depend on it's amplitude as the voltage would be higher and the magnetic field stronger...I agree the higher frequencies contain more energy, as they can do more work per unit wave length on an object.
 

Offline graham.d

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How do radio waves travel?
« Reply #19 on: 06/08/2010 20:11:08 »
I think you maybe thinking about this wrongly. The photons don't emerge serially. As I said, wave-particle duality is not easy to understand. Photons are not long things that have length. If you want to think in terms of a wave (such as a radio or light wave) then energy is proportional to amplitude (actually the square of the amplitude) and also proportional to the frequency. It is hard to think of an example of a radio wave as quanta (or photons) but the theory holds for all em waves. If we go up in frequency to light then we have examples that demonstrate the wave-particle duality. A semiconductor photodetector operates because a photon creates an electron-hole pair in the depletion region of a diode and then the two charges are swept away in the electric field and result in a small current which can be amplified to give a detection signal. With wave theory this could not happen because the energy of the wave is distributed over a wide area. However, it was in reconciling such effects that it was concluded that sometimes light behaves like a wave and sometimes like a particle. The energy of the quanta (the photon) has to be sufficient to trigger the photo detector and this energy is hF as stated. The greater the intensity the more the signal as each photon triggers an electron-hole pair creation and there are more photons per second. An array of such detectors will show discrete positional detection which when taken over a period of time will represent the spread of the incident wave coming in, but nonetheless each detection represents one photon. Increasing the light level increases the number of detections (ie is proportional to the number of photons). The photon appears as a point size particle such that all its energy is concetrated when detected. A wave does not. However the light can be diffracted and can produce interference patterns associated with wave behaviour. There is basically no simple everyday explanation for this. It is a feature of nature that is outside our everyday experience but which can be shown to be the case.

You are clearly thinking deeply about this but you would benefit from reading more about em theory and then basic quantum theory. It is beyond me to give a tutorial on such large subjects on this website.

It turns out that electrons themselves can behave as waves as well as particles. In this case the wave is less tangible than the electromagnetic wave and is a probability amplitude of the electron being in a particular position and this is sinusoidal in nature. Electrons can also be diffracted and produce interference patterns. In can get quite mind boggling. I think someone said something like if you think you understand quantum mechanics you don't understand quantum mechanics. This is not to say you can't go through the equations and make predictions, and if you do it enough you can anticipate results, but it is not something our brains quite accept as normal behaviour. I guess it depends on what is meant by "understanding."
 

Johann Mahne

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How do radio waves travel?
« Reply #20 on: 08/08/2010 19:47:45 »
Ok, I've done some quick snooping, as you suggested.

 You are talking about the energy of a single photon, which has a related frequency and energy.
The tv show I was watching was,I suspect, misleading everyone.The teacher was talking about light but he should have been speaking about photons.
 I will accept that photons are hard to visualize on their own.

 I'm thinking of the energy density of light, and thinking of light as a stream of photons.It most likely is not that helpful to think only of a one photon equation.
  I was trying to figure out what wavefronts would look like from stars that are far away as opposed to stars that are closer.
 I'm Also trying to visualize how  the total wave would  look in a cross sectional area.
I'm also thinking about how to calculate it's energy for that cross section. For example if you are looking at the light of a star in your telescope and it's really far away as opposed to the light of a star that is closer.
  So the light of a faint star but have a lower density of photons. I would guess it's amplitude and magnetic fields cannot be the same as a light from a nearer star, but I'm not sure how this would be affected.
  I don't think your energy calulation here will work , just based on frequency.
 
  Tallying all the photons will give the total energy, but that's would not help me visualize the wave itself.
 
 I don't think it's right to say that a light wave can't be visualized because of quantum effects. If this is the case, then why are there so many different models of atoms around, all drawn recently? Electrons are quantum objects too.
 



 
 

Offline graham.d

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How do radio waves travel?
« Reply #21 on: 09/08/2010 08:51:05 »
I did not say that a lightwave cannot be visualised because of quantum mechanics but just that it is not a complete picture that would describe all its potential behaviour. You can think of light completely as a wave if you like but, for example, it is impossible to explain photodetection or spectral emissions correctlyif you do so. For most optics experiments it would be completely viable to just use a wave model though. As I said before, electrons are also wave-particles but, for these objects, the more common way to think of them is as discrete particles. That they also behave as waves is more tricky to demonstrate but it can be shown that they diffract and interfere in a similar way to light.

It's not "my" energy calculation by the way. Max Planck had something to do with it :-) I agree that you have to use the most appropriate way to calculate the energy, and that for estimating the magnitude of stars, you probably do not need to understand quantum mechanics. Such measurements were done before QM was conceived.
 

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How do radio waves travel?
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