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Electromagnetic Spectrum

  1. Mar 19, 2008 #1
    Hi all. I'm an A Level Physics student and my teacher couldn't really answer this question i had..

    Why can Radio Waves, Micro Waves, X Rays and Gamma Rays go through things (like walls etc) but the others can not.

    My understanding is that they are all just different frequencies of photon wobble??
  2. jcsd
  3. Mar 19, 2008 #2


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    I can't give you a very technical answer, because it's not an area of study for me. Basically, though, it's the properties of the material that make the difference. Visible light will go through translucent or transparent solids, while X-rays won't go through dense stuff like lead.
  4. Mar 19, 2008 #3
    I can understand if it's to do with the increase in energy - therefore gamma is most penetrating but what I really don't get is why radio waves go through most stuff?
  5. Mar 19, 2008 #4
    Matter is mostly empty space, so the question isn't why do some waves can go through matter, but why are there some frequencies where the waves *cannot* go through. Those are are around the frequencies where photon energy closely matches the gaps between the energy levels of electrons. If a light with a frequency around the range of infrared, visible or ultraviolet goes through an atom, there's a good chance that an electron will absorbs it and use the energy to jump up to a higher energy state.

    Molecules also have energy states. If the amount of energy is right, a molecule can absorb a photon and use that energy to vibrate or rotate, so you would have increased the temperature of the material.
    Last edited: Mar 19, 2008
  6. Mar 19, 2008 #5

    What do you mean by radio waves going through most stuff? For example say you have an FM radio wave generated by a radio station. The energy level of that type of wave is low in comparison with the Electromagnetic spectrum we are familiar with. Now let's say you were in a big wooden box with a hand held radio, you still get a signal, so the signal penetrates that box. But some forms of light cannot penetrate that box, is this what you mean but goes through most stuff?

    My theory would be that the wood of the box does not absorb any of the radio waves. If it were a metal box, the electromagnetic energy would want to travel around the outside of the box, and you would get no signal sitting inside of it with your receiving radio.

    This is a good question though, could probably get really complicated. If you shine a flashlight at the box, it won't penetrate right? or maybe it will? A light source has a magnetic wave and an electric "field" perpendicular with it at all times, maybe the wood would block one of those properties?

    I know too that the higher energy levels light ultra, x ray and gamma rays start to become more ionizing than just the longer wavelengths. So these waves are powerful enough to ionize a substance. Then some waves lead will block, but others lead won't block but ten feet of concrete will.

    Also if something is opaque to us, like a black piece of paper or fabric, or even a solid black piece of plastic, visible light won't pass through it, but infrared light may still pass right through it.

    just like in optics how you can have a band pass filter where no visible will pass, infrared will pass, but UV won't. I think it has something to do with the material absorbing the other lights but not the infrared.

    Wow this is an awesome question. This is way out of my area of knowledge though but i would like to see someone answer this.
  7. Mar 19, 2008 #6
    Yeah that's like how a microwave oven works, depending on if the material is a cation or anion, if it is attracted to the wave it will align itself with the positive side of the wave, then when the wave shifts it aligns with the bottom, and thus fast enough it causes friction and heats up.

    some items won't heat in the microwave oven though, materials that aren't conductive like wood, Styrofoam etc.

    Now when you say "If the amount of energy is right", do you mean if it is the same frequency that will resonant a certain molecule or what?

    What is happening when you shine light at say wood, and it doesn't penetrate but a radio wave can go through?

    Sorry for any incorrect terminology it's been a while for this kinda stuff for me.
  8. Mar 19, 2008 #7


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    We have something that addresses optical conductivity in a material in the FAQ thread in this forum. You might want to try there. It at least introduces two important aspects:

    1. A "solid" is different than an atom, and that many properties of a solid does not resemble an atom

    2. the phonon, or lattice vibration, plays an important aspect in the propagation and absorption of EM wave in a solid.

  9. Mar 20, 2008 #8
    The energy of any photon is proportional to the frequency of the e.m. wave. An atom or molecules gains or loses energy by absorbing or emitting one photon at a time. The atom or molecule has certain allowed energy states. If the difference between two energy levels, the one that it's now in and a higher one, happens to be equal to the energy of a photon that is passing through, the atom or molecule can absorb the photon and jump to that higher energy state.

    It's also unstable for it to remain in that higher state, so it drops down to a lower energy state, giving off a photon. When it drops back down, it usually drops the amount needed to give off only infrared radiation, and in random directions. So opaque materials block the light by absorbing the energy and then reemitting it. We don't see it being reemitted because the infrared that comes out is invisible.

    To confuse the issue some more...., solids and liquids and (in stars) very dense gasses, have their atoms so close together and interacting that they modify each other's energy states, causing them to have a large number of in-between energy states that a single atom doesn't have. Therefore, they may be opaque to a whole range of frequencies, rather than merely the frequencies that I described as "just right." So instead of absorbing just this particular red color and just this particular yellow color, etc., they may absorb any light that's within some wide frequency band.
    Last edited: Mar 20, 2008
  10. Mar 20, 2008 #9
    The photon is actually absorbed by an electron, which push it to a higher energy level. How this absorbed photon ( by an electron) make the whole atom or molecule to vibrate?
    How is radiational energy transformed to kinetic energy, which involve the bulk movement of a system of protons and electric cloud ? The photon is absorbed by an electron and this photon somehow vibrate a system of electric charge known as atom or molecule.
    I am actually looking for the exact mechanism.
    Last edited: Mar 20, 2008
  11. Mar 20, 2008 #10
    A covalent bond between atoms can also absorb a photon. Just as electrons do, molecules also have discrete absorption spectra and emission spectra, because their vibration and rotation are quantized. This applies mainly to solids and diatomic gases.

    If you want to search, some terms that I remember regarding the energy states of molecules, how many "degrees of freedom" of motion there are in various cases, and what the effects are on specific heat and various other properties: ... the law of equipartition of energy ... the law of Dulong and Petit ....
    Last edited: Mar 20, 2008
  12. Mar 20, 2008 #11
  13. Mar 22, 2008 #12
    It's really good.
  14. Mar 24, 2008 #13

    Andy Resnick

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    I think you are asking why the electromagnetic transparency of materials varies with wavelength.

    First, from energy balance, the transmissivity is equal to 1- reflectivity - absorption. If we forget about reflectivity for now (reflectivity off the initial surface), the transmissivity is 1-absorption.

    So, the question is now equivalent to "how do some electromagnetic waves get absorbed and others do not?" The reason why I changed the question slightly is because this question is easier to answer.

    The optical properties of any material depend on the electrons. The electrons that are tightly bound to a single atom, the electrons that participate in chemical bonding, and any free electrons. All these electrons can interact with a certain range of energies (frequencies) of the electromagnetic field. That's becasue the energy contained in the electromagnetic field can be used to move electrons from one energy state to another.

    Tightly bound electrons, inner-shell electrons, can only change their energy states by a lot- they interact with X-rays. outer-shell electrons can deal with smaller energy packets, and so they respond to the visible. Electrons in chemical bonds interact with infrared radiation, and so on.

    If a material does not interact with a particular frequency of radiation, because the energy packets of the radiation do not match up with allowable electronic transitions, then the material is transparent to that frequency.

    Also, macroscopic quantities of matter do not have sharp absorption bands- the presence of many 'identical' atoms in proximity leads to a broadening of the absorption band (homogeneous broadening).
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