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Author Topic: ABSORBTION&EMISSION OF ELECTRO MAGNETIC RADIATION  (Read 4159 times)

Offline LionHearT_az

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JUST WANT TO HAVE DETAILED INFORMATION ABOUT IT!!!
PLEASE HELP ME...

ABSORBTION&EMISSION OF ELECTROMAGNETIC RADIOATION BY MATTER
LionHearT
« Last Edit: 09/04/2006 14:54:20 by LionHearT_az »


 

Offline daveshorts

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Re: ABSORBTION&EMISSION OF ELECTRO MAGNETIC RADIATION
« Reply #1 on: 09/04/2006 15:40:26 »
What do you want to know about it?
 

Offline FuzzyUK

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Re: ABSORBTION&EMISSION OF ELECTRO MAGNETIC RADIATION
« Reply #2 on: 09/04/2006 17:01:03 »
Generally speaking - in terms of radiowaves at any rate - the higher you go in frequency, the more is absorbed by surrounding materials.

If you don't believe me stick the cat in the microwave oven and test your meal to see if it is piping hot after being bombarded by 5 Ghz of radiation for 5 minutes. If not satisfied, bung your unusual meal in the gas oven and cook for a further half an hour at Mark 6.

Fuzzy
 

Offline ukmicky

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Re: ABSORBTION&EMISSION OF ELECTRO MAGNETIC RADIATION
« Reply #3 on: 10/04/2006 00:07:52 »
quote:
Originally posted by FuzzyUK

Generally speaking - in terms of radiowaves at any rate - the higher you go in frequency, the more is absorbed by surrounding materials.

If you don't believe me stick the cat in the microwave oven and test your meal to see if it is piping hot after being bombarded by 5 Ghz of radiation for 5 minutes. If not satisfied, bung your unusual meal in the gas oven and cook for a further half an hour at Mark 6.

Fuzzy

But of course as far as microwaves are concerned that is highly dependent on what the surrounding material is comprised of . For instance metal reflects them food absorbs them and they travel straight through glass.



Michael
 

Offline rosy

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Re: ABSORBTION&EMISSION OF ELECTRO MAGNETIC RADIATION
« Reply #4 on: 10/04/2006 00:35:25 »
Caveat: I'm a chemist so most of what I understand about EM radiation comes from what we use it for in spectroscopy rather than a fundamental understanding of what's going on... but anyway...

quote:
Generally speaking - in terms of radiowaves at any rate - the higher you go in frequency, the more is absorbed by surrounding materials.

It's a bit more complicated than that...

Absorption/emission depend entirely on there being an energy level transition between two states at the same energy as the photon to absorb it. So it's always to some extent dependent on what the material's made out of.
Cosmic rays just basically go through everything (through the earth and out the other side), gamma rays are emitted by nuclear reactions (such as fision/fusion) x-rays are emitted and absorbed by electron transitions between an atom's lower energy electron orbitals and the higher energy ones (or, sometimes, are absorbed by promoting an electron so much that it leaves the atom entirely, giving an ion and a free electron).
UV is absorbed and emitted by transitions between electron orbitals which are closer together in energy, often now molecular rather than atomic transitions. Visible light is still lower in energy and largely linked to electron transfers- examples I can think of are between transition metal d-orbitals, charge-transfer interactions between species often in solution, and between conjugated pi-electron systems such as in carotene, lycopene, chlorophyll...
Moving down now into the infrared region, it's more to do with exciting different bending and stretching modes of molecules. Microwaves tend to excite rotational states, this is important in microwave ovens since they increase the rotational energy of water which is then dissipated by thermal transitions though the food.
Radio waves are used in NMR/MRI applications because they change the spin-states of the atomic nuclei.

Basically all the different types of energy that materials can have are quantised and if a photon energy corresponds to the transition between two quantised states then it will tend to be absorbed. Or, if there are a lot of excited states present (because the material is hot or has been excited by some other means) then the transition of the material to the lower energy state will be accompanied by the emission of a photon.

If a material has many energy levels close together (as in a metal, where there are loads and loads of energy levels of electrons close together) then it will absorb/emit lots of different wavelengths). I *think* the reflection thing with metals is due to the photons being accepted and then reemitted going the other way but no doubt someone who Knows Stuff will correct me on that one.
 

Offline Soul Surfer

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Re: ABSORBTION&EMISSION OF ELECTRO MAGNETIC RADIATION
« Reply #5 on: 12/04/2006 23:53:31 »
Rosy has given some useful indications (but they are not without errors) but I would like to offer a physics point of view which is rather more general.

The absorption of electromagnetic radiation by matter depends on that matter interacting with the radiation by having energy states avalable to it for example glass is transparent to light just because it does not interect with the light  however materials like thick PTFE sheets don't intract with the light by absorbing it but do scatter it so they are translucent. Other materials for example metals are conducting and these tend to reflect electromagnetic radiation particularly when they have smooth polished surfaces.  Semiconductors tend to absorb energy except for very specific frequencies.

That brings us to a bit about the wavelength of the electromagnetic radiation.  Longer wavelengths have lower energy quanta and tend to interact generally with materials on a scale commensurate with their wavelengths so radio waves and microwaves generally interact with matter on a large scale light and ifra red on a smaller sacle  X rays are getting down to an atomic scale and can pass through the electron cloud of atoms with a low probability of interecting with them and so materials become more transparent, gamma rays penetrate even further but no known high energy electromagnetic waves pass right through the earth. that honour is left for neutrinos and any small dark matter particles which are not electromagnetic.

Finally let me work my way up the frequency range lookig at the interactions that cause electromagnetig radiation to be absorbed

Radio waves and microwaves    
   bulk conductivity propeties of the material  presence of water etc NMR is a very spacial case that applies to precise frequencies and magnetic fields  ESR  (electron spin resonance is a similar special case at highrt frequencies.

Infra red radiation
   This is the range of frequencies involved with the motions of atoms in solids liquids and gasses at room temperatures and almost all materials absorb energy very strongly in this range.

Light
   This is getting into the range of energies needed to remove electrons from atoms and so materials tend to interact only at very specific frequencies unlke infra red where there are often very many closely related frequencies for interections

X rays  
   This is getting higher than the enegies needed to remove even the innermost electrons from atoms but well below those needed to distub nucleii and many materials are transparent or have low absorbtion at these frequencies.

Gamma rays  
   These tend only to interact with atomic nucleii and rarely with individual electrons causing nuclear transitions recoil and ionisation.



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Offline LionHearT_az

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Re: ABSORBTION&EMISSION OF ELECTRO MAGNETIC RADIATION
« Reply #6 on: 19/04/2006 05:38:51 »
THANX FOR UR ANSWERS...LET'S CHANGE QUESTION...
EMISSION AND ABSORBTION OF LIGHT BY SUBSTANCES AND METHODS DEPENDS ON IT...

LionHearT
 

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Re: ABSORBTION&EMISSION OF ELECTRO MAGNETIC RADIATION
« Reply #6 on: 19/04/2006 05:38:51 »

 

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