Wavelength passing through matter

[Jlao]

Radio Wave, visible light or even Gamma-ray are just some different wavelength of 'electromagnetic radiation' (right?).

 

So for sample, lets take a 10cm (3.93inch) Concrete wall,

Gamma-ray should be able to pass through it because of his small 'size'

Visible-light definitively can't pass though it.

But why a Radio-wave is able to pass though ? knowing that it's bigger than the visible-light.

 

Thank you in advance for your help.

Edited February 19, 2015 by Jlao

[StringJunky]

It's not size that matters but energy. Visible light has the right range of energies to interact with the concrete components so they get absorbed. The longer radio waves and very short gamma rays pass through because they don't have enough energy and too much energy respectively.

 

Take transparent glass: when a visible light photon meets a molecule it has insufficient energy to excite an electron in its outer shell to lift it to the next quantum state and so it gets emitted until it reaches another one and the same thing happens again until it passes out the other side. If you sent a UV photon it would get absorbed because it can interact with an electron in glass, so, to a UV photon glass is opaque, like concrete is to visible light.

Edited February 19, 2015 by StringJunky

[swansont]

It's also the material. A wall of metal will react differently than a wall of concrete. The metal wall has electrons that are free to absorb the EM radiation, while the concrete (in general*) does not.

 

 

 

*there is a material known as "conductive concrete", which has conductive materials mixed in. Used e.g. for EM shielding and electrical heating for de-cicing.

[Sensei]

One needs to be very careful to distinguish bouncing from walls so many times it appears to be "passing through" from real transparency.

It would require using directional transmitter with directional receiver, experiment outdoor (to not have reflections from walls).

 

[Klaynos]

One needs to be very careful to distinguish bouncing from walls so many times it appears to be "passing through" from real transparency.

It would require using directional transmitter with directional receiver, experiment outdoor (to not have reflections from walls).

 

You can do free space microwave transmission and reflection experiments inside (and optics, IR and terahertz of course). You've just got to be careful.

[Sensei]

I was doing it in 2007 with 2.4 GHz.

With directional transmitter I could receive signal at 100 meters of air with ~100% quality.

After moving 1 cm behind concrete wall (few cm size, really small), signal couldn't pass through it..

 

Pointing two directional transmitter and receiver at 100 meters distance so they see each other = nightmare.

Now I think I should attach to them laser pointers to see exactly where they're pointing.

 

I also hate adjusting satellite receiver.

1mm difference and there is no signal from satellite.

You don't know where is satellite (just by looking at neighborhood set up their dishes) and have to do it blindly..

 

Wifi 2.4 GHz/satellite transmission is really badly working/not working when there is heavy rain or snow.

Edited February 27, 2015 by Sensei

[studiot]

I'm not quite sure why the OP has not returned since posting and the collection of replies are pretty helpful.

 

However there is one missing fact.

 

It is not strictly true to say that light does not pass through a concrete (or any other) wall.

 

For any material there is what is known as the attenuation coefficient.

 

http://en.wikipedia.org/wiki/Attenuation_coefficient

 

All radiation, including non wave particles like alpha and beta and sound, the attenuation of the radiation depends upon the thickness of the obstacle.

 

For most materials this is a non linear exponential relationship with a 'half thickness' similar in concept to the half life of radioactive substances.

This 'half thickness' is the thickness that will reduce the radiation by 50%.

 

The difference between coefficients for different materials and for differents forms of radiation is due to the specific differences in the interaction between the radiation and the material, as others have noted.

[Klaynos]

I was doing it in 2007 with 2.4 GHz.

With directional transmitter I could receive signal at 100 meters of air with ~100% quality.

After moving 1 cm behind concrete wall (few cm size, really small), signal couldn't pass through it..

 

Pointing two directional transmitter and receiver at 100 meters distance so they see each other = nightmare.

Now I think I should attach to them laser pointers to see exactly where they're pointing.

 

I also hate adjusting satellite receiver.

1mm difference and there is no signal from satellite.

You don't know where is satellite (just by looking at neighborhood set up their dishes) and have to do it blindly..

 

Wifi 2.4 GHz/satellite transmission is really badly working/not working when there is heavy rain or snow.

I've done around 20 m paths at a similar frequency using collimating mirrors. We gave up using lasers for alignment. Over longer distances I would be very tempted to try them again.

[swansont]

 

All radiation, including non wave particles like alpha and beta and sound, the attenuation of the radiation depends upon the thickness of the obstacle.

 

 

Charged radiation is not attenuated, i.e. it does not have an exponential decrease in penetration. Alphas, in particular, have a fairly well-defined, energy-dependent penetration depth, since they are massive and don't scatter very often.

[studiot]

 

swansont

Charged radiation is not attenuated, i.e. it does not have an exponential decrease in penetration. Alphas, in particular, have a fairly well-defined, energy-dependent penetration depth, since they are massive and don't scatter very often.

 

 

I didn't say the relationship was exponential.

But it is definitely untrue to say that charged radiation is not attenuated by the interposition of matter its the path, or that the reduction in measured radiation is not a function of the thickness of the matter impeded path.

Agreed my wording was not perfect, but yours was no better.

 

 

studiot

For most materials this is a non linear exponential relationship with a 'half thickness' similar in concept to the half life of radioactive substances.

This 'half thickness' is the thickness that will reduce the radiation by 50%.

 

Edited February 27, 2015 by studiot

[swansont]

I didn't say it the relationship was exponential.

But it is definitely untrue to say that charged radiation is not attenuated by the interposition of matter its the path, or that the reduction in measured radiation is not a function of the thickness of the matter impeded path.

 

When you lump all of it together, there is an implication that it behaves the same way. And for charged particles the reduction is not a simple function of the thickness. If you have a thin foil, almost all of the alphas will penetrate. Adding an incremental layer will show little attenuation, until some point, when it drops dramatically and quickly goes to zero. Adding material after that has no effect, since you've stopped all the alphas. It's roughly a step function.

 

See figure 6

http://www.alpharubicon.com/basicnbc/RadiationPenetration.htm

[studiot]

 

swansont

It's roughly a step function.

 

So it is a function, and roughly is not exactly.

 

 

studiot

Agreed my wording was not perfect, but yours was no better.

 

I have already agreed I could have said that the attenuation is a function of both material, thickness and the radiation concerned and that would have been better.