Why do different frequency EM Waves behave different ?

JPC

Hi, i was wondering
Sorry if i dont have the right words, i study in France

Why are Microwaves better for heating up food ?
I mean since E=hv, these should contain less energy
Right now, i know vaguely, that atoms absorb photons where the energy can make it go from one of its characteristic Energy states to another.
Is it that lower energy photons are compatible with an energy state leap of more atoms ?

And how can some Lasers actually decrease the temperature (energy) of a material ?
I have only seen Leaps from E0 (fundamental status) to E=0
How does this work ?

And with radios used in medecine to see bones.
Why do higher frequency waves 'go through' more materials ?
Is it because they do not correspond to many energy status leaps of different atoms ?
If this is so, we would have used very low waves too, so doesnt make sense

And, how does refraction and reflection work in quantum mechanics ?

IF this is all too long for you to answer, is there a website which can explain we all about waves in quantum physics (without going into very hard details, i have just finished high school)

malawi_glenn

Water have high absorption in the Microwave part of the spectrum, compare with absorption lines of atoms, only certain wavelenghts of light can be absorbed.
http://en.wikipedia.org/wiki/Absorption_spectrum
http://www.lsbu.ac.uk/water/vibrat.html
http://users.rcn.com/jkimball.ma.ult...nSpectrum.html


And about radio waves in medecin, it is not the radio waves that makes the image, but the radio frequence correspond to the frequency of the nucleis spin, so that the spins of the nuclei will align with the radio-field. Then one uses a big magnet to localise where these nuclei are in the body. The nuclei one wants to see are the hydrogen in water, and bone does not contain water. Read more about this here: http://en.wikipedia.org/wiki/Nuclear_magnetic_resonance

DaleSpam

There a lot of different medical applications of EM energy at different frequencies. It basically depends on what you want energy delivered to or from (or what you want to avoid).

ELF-SLF, good for exciting nervous and cardiac tissue, used in pacemakers, defibrillators, EEG, ECG, and DBS

VLF-MF, good for not exciting nervous tissue even with large currents, used in RF ablation and electrosurgery

HF-UHF, not absorbed much by body tissues, corresponds to proton Larmor frequency at reasonable field strengths, used in MRI and communication

Infrared, widely absorbed emitted and scattered by tissues, used in thermometers and pulse oximeters

Visible, widely scattered by tissues, used in OCT, external (visual) diagnosis, and optical activation targeted drug delivery

Ultraviolet, absorbed by tissues and can be ionizing, used for sterilization, neonatal jaundice?, vitamin D therapy

X-rays, absorbed by bone, used for imaging (x-ray, CT)

Gamma-rays, not absorbed by much, used for imaging (nuclear medicine)

JPC

oh, so a Microwave is just sending waves that are absorbed by water
hopefully its everywhere in our food.
Puting dry materials in microwave may have limited effects then ?
and why metals react violently to microwaves ?

Also, i was wondering, how come some objects are dark or black ?
- i mean, if the photons are not absorbed they should continue their way through the material, like water (transparent)
- and when they are aborbed , the atoms must go from a energy state to a higher one, and then re-emit photons to come back to their Fundamental energy state.
So it seems that black/dark materials absord photons, but only release the energy through heat ?

malawi_glenn

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

the colour of a material is the one that the material reflect/does not absorb. A black surface absorbs everything, and a white nothing, and a red everything except red.

JPC

yes, but once a black material has absorbed the photons, he must re-emmit photons to comeback to its fundamental energy level ?
are this emmited in non-visible ?

What I was trying to understand is the quantic difference between transparent and opaque objects .
and how does the atoms reflect the photons they don't absorb? I thought they would just pass through

DaleSpam

Usually it is re-emitted in the infrared

malawi_glenn

exactly, solid bodies behave different than single atoms (gas), solid bodies can in many cases be approximated by the "Black Body"-concept in thermodynamics.

JPC: An object is transparent if it for a certain wavelength "kills" the beam of photons, i.e absorb it, and that the 'optical depth' is much smaller than the size of the body.

for example, my bible here on my desk is opaque to visible light, since I cant see right through it. But if I would have access to a X-ray source, the X-rays would go right through it.

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

JPC

I dont understand
i thought transparent objects dont absorb anything

Like for example, if you have a white solid and, transparent solid, a black solid.
and if we consider a beam of white light

- In the black one, the photons are absorbed and the atoms move to a higher state of energy, and then release that energy by emmiting non-visible frequency photons
- In the White one , none are absorbed, but how come the photons change direction, and are emited in every direction ? is it that actually the atoms absorbs them, but re-emmits them straight away with the same frequency ?
- And now what happens in a Transparent object exactly to photons ?

malawi_glenn

Transparency is relative a certain wavelenght, the EM-spectrum is more than just the colours we see with our eyes.

JPC

i dont understand what you mean
any webpages that explain this in details ?

i know that the visible bandwidth is very narrow, but "Transparency is relative a certain wavelenght"

DaleSpam

Google "absorption spectrum" especially the absorption spectrum of water.

malawi_glenn

Absorption is a function of the wavelenght, i.e the transparency is afunction of the waveleght. And this function differs between the different materials.

JPC

but what inside a atom or material characterizes if it will be transparent or opaque ?

i mean for absorption, we know that it is the different characteristic energy levels of the atom with E=h*v
but for transparency ?

ZapperZ

Hum... I'm beginning to feel like a parrot, but you might want to read our FAQ thread in the General Physics forum.

Zz.

JPC

But its not easy to find the specific Thread
the Website should offer a 'search posts containaing the text you want' utility

But i was wondering about one application for the millitary
for spies : a material opaque to visible light, but transparent to IR
and they could wear IR to spy safely behind a wall made of this material

are there any more realistic applications ?

ZapperZ

The FAQ is one of the Sticky threads at the TOP of the General Physics forum. What is there to find?

Zz.