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The electromagnetic spectrum is wide, from radio waves to gamma rays. But why can materials obsobe only a specific wavelength? For example why can you receive a radio wave with an antenna but not light or x-rays. They are all electro magnetic radiation.

I am not a doctorate so an intuitive answer would be welcomme.

And am I correct if it has sommething to do with the fact that we look at somme "waves" as particles?

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    $\begingroup$ Electrons are like form fitting violin cases when absorbing energy from something like a photon. Just like how if the violin is too small or too big it won't fit in the case, if the photon isn't of the correct energy level, such as being too high or low, the electron can't absorb it. "a radio wave with an antenna but not light or x-rays" You can, but antennas for light or x-rays are much more difficult to make. $\endgroup$
    – DKNguyen
    Sep 13, 2022 at 13:45
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    $\begingroup$ @DKNguyen why can't it? $\endgroup$
    – user253751
    Sep 13, 2022 at 13:49
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    $\begingroup$ @user253751 "why can't it?" Because it can't. That's the physics. Abandon all notions of human intuition, experience, and analogies here because they won't work. You need to take the deep dive at this point. The best that can be done without a deep dive is to give examples that obviously wouldn't make sense if electrons around atoms could absorb any energy. webassign.net/question_assets/buelemphys1/chapter27/… $\endgroup$
    – DKNguyen
    Sep 13, 2022 at 13:58
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    $\begingroup$ @DKNguyen I bet the quantum mechanics guys will come up with a good underlying reason one day, if they haven't already. e.g. something like the electron does get excited, into an unstable state that lasts half a femtosecond before decaying back to the one it came from. $\endgroup$
    – user253751
    Sep 13, 2022 at 14:19
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    $\begingroup$ @user253751 The end of the explanations to a never ending chain of "why?" questions is like stomping on someone's toes: The original problem doesn't go away as much as you just get distracted by something else. $\endgroup$
    – DKNguyen
    Sep 13, 2022 at 14:27

2 Answers 2

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Electromagnetic (EM) waves are energy oscillations. Materials absorbed the incoming energy oscillations by different modes, including oscillations of electrons in orbitals, vibrations of ions, vibrations or rotations of molecular dipoles, and oscillations of a collective sea of free electrons. Various types of materials have various types of these modes. For example, metals essentially have no ion oscillation or dipole oscillation modes. Each mode has its own resonance frequency depending on various factors associated with the mode. For example, the ion oscillation resonance frequency depends on the masses of the ions.

The critical factor is the resonance frequency. Think about a swing. When you try to input energy with a frequency above its natural resonance frequency, the swing will not respond. In this same regard, when you input EM waves above a resonance frequency for a mode, the mode will not absorb it. Visible light transmits through window glass because the resonance frequency for ion modes is below visible. Visible light does not transmit through metals because the resonance frequency for free electron oscillations is in the UV or above.

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Elucidating on the basis of the usual particle-wave duality: the other answer mentioned resonance condition on the light frequency, but I personally like to think about it in the particle sense.

Light in this sense is nothing but a bunch of particles. If your light source has only one wavelength (eg. monochromatic), then all light particles have the same energy. Now, when something absorbs the light, this energy must be taken up by the absorbing matter.

A peculiarity of quantum mechanics is that in many cases, the matter can not have any energy, it can only have certain discrete energy values. Before the light hits the matter, it has a certain amount of energy (it is in the so-called ground state), and after it absorbs the light particle, it will have more energy.

Here's the catch: since the matter can only have pre-defined values of energy, it can only absorb light that has a wavelength that corresponds to an energy difference between the ground state and the level that it will get excited to.

Note that this is exactly the same description as the one based on the resonance condition with the frequency of the light. I prefer this one because for me this picture is easier to generalize for example to the cases of multiphoton absorption.

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  • $\begingroup$ Electronic excitation is entirely different from, cannot be substituted for, and does not define resonance cutoff frequencies for absorption by polarization modes. $\endgroup$ Sep 14, 2022 at 13:08

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