Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

EM penetration of physical objects

  1. May 12, 2004 #1
    Why can just about all frequency/wavelength electromagnetic waves penetrate through physical objects (even if only a thin piece of wood or something) but light cant?
     
  2. jcsd
  3. May 12, 2004 #2

    Gokul43201

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Light does too - to some small extent. Light is just an EM wave whose frequency/wavelength lies within a certain range.

    For example, the thickness of certain very thin Electron Microscopy samples can be estimated by their color. A really thin slice of silicon (or anything else, for that matter) is transparent.
     
  4. May 12, 2004 #3

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    <Holds up a clear piece of transparent glass>

    Look! Light penetrates throught it! So whatever can you mean by that? Unless you do not consider glass as a "physical object".

    Zz.
     
  5. May 12, 2004 #4

    krab

    User Avatar
    Science Advisor

    On one level, the question does not make sense, since light IS an electromagnetic wave.
    Did you know that submarines cannot communicate by radiowaves, since water absorbs them, even though, as we know, light can penetrate water?
    If I am in a tunnel, I cannot get any radio reception. Maybe you are confused about diffraction? It is true that e-m waves cannot penetrate metal, so if I put a radio in an enclosed metal box, it doesn't work. But if I put it on one side of a metal wall, it will work. The reason is that the radio waves are long compared with the formation, so they can just go around it. Light waves are very short compared with a mm, and so appear to travel in straight lines.
    HTH
     
  6. May 12, 2004 #5

    Nereid

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Welcome to Physics Forums raymes_k!

    As you've seen from the previous posts in this thread, your question could have been worded rather better.

    It's certainly true that solid materials vary greatly in their ability to transmit light, and EM radiation of other frequencies. The same is true of liquid materials. What do you think about gases?
     
  7. May 12, 2004 #6

    LURCH

    User Avatar
    Science Advisor

    Still, the question is, on the whole, valid. For example, in an in it is true that light in the visible spectrum and will pass through glass, and that is why am I can see out the windows of my house. In fact, it is the very fact that the windows are transparent to light in the visible spectrum that makes them an exceptional part of the wall. Yet, if I put my radio in a room with no windows, it will still receive a signal x-rays will pass through the wall, and a person with "heat vision" or "thermal vision" goggles will be able to see me through the wall. The same will hold true for Sheetrock, drywall, plywood, cloth, even (to a lesser degree) stone or brick. A strong enough heat source can be detected by a thermal scope even through a solid slab of concrete. A visible light, no matter how bright, won't. Is it simply coincidence that so many materials in our everyday life are opaque to light in the visible spectrum and translucent/transparent to the majority of the rest of the spectrum?

    For organic materials I could see some connection with the fact that organic sells are specifically designed to trap light in the visible spectrum, because that is the spectrum in which our Sun produces the majority of its radiation. But what about the rest? Setting aside metals and crystals, there is still an awful lot of material that fits this description, isn't there?
     
  8. May 12, 2004 #7

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    But krab is exactly correct in asking if one is confusing "diffraction" with "penetration". Just because you can get a radio signal sitting behind a conconcrete wall does NOT mean that radio signal actually when THROUGH the wall. Radio waves can have wavelength of the order of MILES. A house is NOTHING compare to this. Diffraction alone makes it possible for one to get a signal inside a house/behind a wall, etc. One should not be mislead into thinking that the radio wave actually penetrated through the wall.

    There is a whole section of condensed matter physics that study optical conductivity through materials. This range from passing microwave, visible light, infrared, all the way to x-rays through different materials. Techniques such as FTIR and Raman scattering are a few such examples. So this isn't an "unknown" phenomena. We have a very good idea on why certain materials are transparent over a certain range of wavelengths. There isn't anything special about visible light. I can always design a material that is transparent to the visible spectrum but completely opage others (eg: ordinary glass - transparent to visible light, opague to UV). So why pick on visible light only?

    Zz.
     
  9. May 12, 2004 #8
    first of all - thanks to all the arseholes who picked on the wording of my question - most probably cause its my first post -- doesnt mean im an idiot. i realise light can penetrate glass (duh) and some other materials but as lurch says, is it merely coincidence that so many materials are opaque to light. the diffraction argument is invalid IMO - although most EM waves (those of lower frequency) cannot pass through a thin sheet of metal, if i were in a room with no windows (as lurch says), i would still recieve radio waves. No matter how the radio waves are getting to me, they must be PENETRATING somewhere. the diffraction argument does allow radio waves to be transmitted to places where they would not generally recieved but its not relevant here
     
  10. May 13, 2004 #9

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    .. and you are rather naive into thinking that we should be able to read your mind into guessing what you actually meant. One major lesson that one learn in studying physics is that words and phrases have EXACT and UNAMBIGUOUS meaning in physics, often with accompanying mathematical formulations. Your question was taken at face value, nothing more and nothing less. To try an "interpret" what you actually meant would cause confusion and we would be accused of putting words into your mouth - or would you rather we do that?

    The question I asked in my last posting stand - what's so special about visible light anyway that you have to pick on it? I can find more things that can stop UV and IR but still let visible light pass through. You simply think that there is something more significant simply because you notice more easily when light is stopped. You will need extra sensory ability to notice when UV and IR and X-ray and everything else are stopped. So just because you notice more of it doesn't mean there really is more of it.

    If you are REALLY serious into figuring out this, then I suggest you look up the area of study that I have already pointed out. Figure out why the phonon structure in a particular material plays a major role in determining if that material will be opaque or transparent to a particular EM wavelength.

    Zz.
     
  11. May 13, 2004 #10

    LURCH

    User Avatar
    Science Advisor

    That was going to be my next question. Is that really true, that there are more things that are opaque to visible ligth than vise versa? I mean, it would seem tio make sense, what with the shorter wavelength and all, btu I never heard if it were actually true or not.

    That might negate my next idea, I was speculating that the propensity of materials opaque to visible light might have roots in solar system formation. The radiation we call "visible" is the same radiation that played a roll in causing certain materials to form the clumps of debris no which we live. But if the whole hpenominon does not exist, and things simply get more opaque as one moves up the spectrum, then bugger it all!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: EM penetration of physical objects
  1. EM wave penetrates? (Replies: 9)

  2. EM penetration (Replies: 1)

  3. EM waves penetration (Replies: 2)

Loading...