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I would like to know the relation between penetrating ability and the frequency of a wave. For example, gamma waves have high frequency and high penetrating power: intuitively I imagined this as attacking a target rapidly in quick succession and managing to get through.

But the penetrating ability is also a feature of low frequency waves - the usual telecommunications spectrum. What is the intuition behind this? Are there other factors like the field which play a dominating factor here?

Edit: The question is in reference to the permeable zone indicated in this figure.

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3 Answers 3

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That's a complicated question because different frequency waves are absorbed/scattered by different mechanisms and in different media. Have a look at http://en.wikipedia.org/wiki/Mass_attenuation_coefficient as this contains some introductory information. Actually that article describes X-Ray absorption in some detail but glosses over light and radio waves.

You specifically mention radio and low frequency waves and http://en.wikipedia.org/wiki/Radio_propagation gives some discussion of these. Low fequency EM is generally too low energy to be absorbed by exciting molecular transitions and instead it's absorbed by interaction with the electrons in whatever it's passing though. generally speaking the more conducting the media the faster the radio/VLF waves are absorbed.

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Hi John, my question was specifically in reference to [this link] (fbcdn-sphotos-a.akamaihd.net/hphotos-ak-ash4/…). There is a permeable region in the low frequency area. –  Bravo Mar 9 '12 at 13:26
@Shyam see my answer. The confusion is between air and solid objects, which react to EMR differently. –  Manishearth Mar 9 '12 at 13:34
Oh, now I see it, John. Thanks. By the way, is this Mass-attenuation coefficient thingy empirical/experimental only? Hasn't research been done to quantify this behaviour for different EMR and different objects? –  Bravo Mar 9 '12 at 14:15
This isn't my area of Physics (I'm a colloid scientist) so I'd have to do a literature survey, aka a Google of arxiv.org, to see what has been done. I suspect it's the sort of thing that's easy to estimate theoretically but hard to calculate with any precision. For real applications I imagine the attentuation would always be measured. –  John Rennie Mar 9 '12 at 16:55

This is more of an addendum to @JohnRennie's answer, outlining your confusion a bit more clearly:

Normally, the gamma-rays-more-PP is for metals etcetera. These have free electrons and the electrons can absorb pretty much anything. Gamma rays, being more energetic, preserve themselves much better that Xrays, radiowaves, etc. Concrete is a composite, so I don't know exactly what goes on inside it, but it's probably something similar.

On the other hand, gas molecules in the air can only absord fixed amounts of energy (above the ionization energy, they can absorb anything)? Since $E=h\nu$, fixed energy $\implies$ fixed frequency. These fixed frequencies are mostly in the UV/Xray/gamma ray range for air. There are enough of these fixed frequencies(due to orbitals, etc) that they basically black out most of the the Xray spectrum. Aside from that, the reaction $3O_2 \overset{h\nu}{\rightleftharpoons} 2O_3$ absorbs UV light in the ozone layer.

On the other hand, radio waves aren't strong enough to be absorbed by an atom, they are probably below the quantum of energy for the atomic system. So they pass through space unhampered, and since they diffract easily, we get the extra benefit of non-directional nature. They can bend around large metal objects as well due to diffraction.

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I did an experiment which is interesting to mention. I laid light on a water bottle and car oil. Light was barely passed through the oil but clear through the water bottle as expected. Then I put both bottles in front of my clinical X-ray and took a shot. Interestingly water was opaque way more than the oil! So first suggested answer to this duality is due to higher frequency of X-ray compare to light , X-ray sees the nucleus and therefore density whereas light sees the outside orbital electrons. But then I remember ultrasound which is against this theory . The higher the frequency the lesser the penetration! What?! Somebody please help.

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Ultrasound uses acoustic (sound) waves, not electromagnetic waves; sound propogates through vibrations in the medium, as mechanical motion. Absorption of EM is a function of the absorption spectra of the material being bombarded, so since oil and water have different densities and chemical compositions they will absorb different frequencies. –  apraetor May 29 at 6:31

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