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Penetration Perturbances

  1. May 25, 2010 #1
    I couldn't sleep the other night because I was thinking about photons and their ability to penetrate materials. It can be argued that humans are photon detectors. This is true in more than just the sense of visible light because photons don't just enter our pupils. As an example, our bodies can detect x-rays (our bones stop them). But we can't form an x-ray map of our surroundings, as we can with visible light, unless computers assist us.

    Imagine for a moment if our eyes were unable to detect visible light. For some reason we evolved to detect x-rays. If I were in a building, away from all windows, what would I see? Assume that all light sources (in this case the sources would emit x-rays) were turned off. Since x-rays have greater penetrating power than visible light, would I still see the Sun if I were inside a building? If one had eyes able to detect x-rays would there be a need to make artificial x-ray sources?

    But then I thought about much longer wavelengths. As you know, if you step outside of a building and shine a flashlight against the wall, the photons will not pass through the wall and be detected by any humans inside. Interestingly, I asked, "Why can radio waves pass through the wall?"

    Since radio waves have less energy than light waves, how are they able to penetrate materials that absorb and reflect visible light? I used to think that radio waves were made of photons, but I read recently that they are not. It just seems logical to assume that the carriers of electromagnetic radiation, the photons, waved at all wavelengths.

    Since radio waves are not made of photons (are they made of electrons?) why are they put on the same chart as visible light, ultraviolet, x-ray, gamma-ray, etc.?
  2. jcsd
  3. May 25, 2010 #2
    Radio and Visible light are exactly the same phenomenon.

    Long radio waves can be explained "classically" because the wavelength is so much larger than the size of atoms, and because they have low energy: you have many many many individual photons each very subtle so you only see the overall effect. The atoms of the radio antenna don't stop and absorb a photon and change state; instead, quantum effects mix displacements of the free electrons with the photons and when it all comes out in the end you say "electric field" etc.

    Short visible waves don't do all that because they are very small compared to the atoms. Your eyes work by having a molecule stop (absorb) photons which causes a change to the electron's energy level in the atom. Visible light's energy is a good match for the available energy levels in organic molecules... so no coincidence that we use it to see with!

    Just at the edge of visibility and into UV, individual photons knock individual electrons out of a metal, which is what lead to the realization that photons exist. Radio waves, in huge numbers, gently sway the electrons in the metal instead, like water molecules in the surf making small seashells dance on the edge of the beach.

    The two things are extreme ends of a continuous range of phenomena. Just like water is "a liquid" in bulk, but individual hard and solid atoms on another scale. Radio and Visible light is like the difference between a pile of sand and a large slab of stone. Different things happen if you drop each one onto you.

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