Transparent matter

Discussion in 'Physics & Math' started by Magical Realist, Feb 6, 2022.

  1. Magical Realist Valued Senior Member

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    How does light pass thru transparent matter like glass and plastic? It seems like the photons would be obstructed by atoms as in normal matter.
     
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  3. DaveC426913 Valued Senior Member

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    Photons don't interact with matter as if they're little solid balls. They interact by being absorbed - usually into molecular bonds. If the photons aren't of the right frequency to be absorbed, they just pass right through.
     
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  5. exchemist Valued Senior Member

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    To add to Dave's reply, glass for instance is transparent to visible light but has absorption bands in the infra red (due to the bonds between the atoms being set in vibration) and also in the ultra violet (due to electrons being kicked into higher energy states). So at these frequency ranges glass is opaque.

    Spectrometers working in these regions need to avoid the use of glass. In IR spectrometers, for instance, it is common to use sample cells made of rock salt. You have to be very careful not to touch the windows with your fingers, as the moisture from them will erode the salt surface.
     
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  7. DaveC426913 Valued Senior Member

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    Not to mention that the particle parts of an atom (nucleus and electrons) are about 1/100,000th the volume of an atom* - the other 99.999% being vacuum, so one might expect only one in every 100,000 passing photons will physically intersect any physical part of the atom. And even that doesn't take into account that the molecules in a solid are not cheek-by-jowl but have space between them.


    *a pea in a stadium
     
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  8. Jonathan Ainsley Bain Registered Member

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    One often hears of photons having frequency,
    but frequency of what exactly?
    Frequency is just an event over time.
    But what actual event has that frequency?
    I've never had heard a satisfactory answer to this question.
     
  9. DaveC426913 Valued Senior Member

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    A photon can be described as an excitation of the electromagnetic field. See the simple diagram below.

    They propagate at the speed of light and the level of excitation determines their wavelength.
    Frequency is just 1/wavelength.


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  10. Jonathan Ainsley Bain Registered Member

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    well that does not answer the question
    because the magnetic field is said to actually be
    the photon too - which is - you must admit - paradoxical
    at least - but i am not going to argue any further
    as you answered with a quote instead of in your own words
     
  11. exchemist Valued Senior Member

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    That's not quite right. The photon is not a field. It is an excitation of the electric and magnetic fields. Just as the ripple on a pond caused by dropping a pebble into it is a disturbance of the water surface. The ripple is not the water surface, but an excitation of it.

    The paradoxical bit, in the case of the photon, as with other quantum scale objects(e.g. electrons, protons etc), is that it has features both of a wave and a particle.
     
  12. DaveC426913 Valued Senior Member

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    No I didn't. I wrote that.

    The diagram has its own words. You may heed or ignore those as you see fit (though I'm not sure why you would dismiss a quote anyway).
     
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  13. James R Just this guy, you know? Staff Member

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    Err... frequency is the speed of light divided by wavelength (\(v=f\lambda\)). You're thinking of the period of a wave.
     
  14. James R Just this guy, you know? Staff Member

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    The frequency is a wave-like property of the photon, which acts in some respects like a particle and in some respects like a wave. It is related to the wavelength and the speed of light (see previous post).

    Classical waves (e.g. water waves, sound waves, etc.) behave in particular ways that sometimes depend on the frequency of the wave. For example, waves reflect, refract and diffract in predictable ways that depend on the frequency. The frequency of a classical wave is the time between two wave "crests" passing a fixing point in space as the wave travels along.

    In experiments involving light (photons), things like refraction, diffraction and so on behave in similar ways to classical waves, so it's reasonable to assign a photon a corresponding frequency.

    Before the quantum revolution in physics, light was described as a classical electromagnetic wave. In that picture, the idea of a frequency has a straightforward interpretation. In the quantum picture, photons can also be described as a wave (technically a sort of probability wave), with the same frequency.
     
  15. DaveC426913 Valued Senior Member

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    I should have just said frequency is inversely proportional to wavelength.
     
    Last edited: Apr 12, 2022
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