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How light travels through glass

  1. Dec 7, 2009 #1

    I want to know how light travels through glass. As per my understanding of physics, a photon is absorbed by an electron and the electron goes to a high energy orbit and returns to its original state after a moment and emits another photon. This is how light travels in glass.

    But how come the photons manage to go maintain their direction?

    Thanks in advance!!
  2. jcsd
  3. Dec 7, 2009 #2
    Re: Transparency!!

    no glass is a solid and hence you must use the band description of energy levels:
  4. Dec 7, 2009 #3
    Re: Transparency!!

    Sorry!! I didn't understand your reply!! What about water? How does light travel in water? that too in straight lines?
  5. Dec 7, 2009 #4
    Re: Transparency!!

    Photons from the visible part of the spectrum does not "travel through" glass in the way you proposed.

    In water, visible light, is absorbed and re-emitted. This re-emission goes in all directions, this is why there is dark at the bottom of the ocean...
  6. Dec 7, 2009 #5
    Re: Transparency!!

    ohk.. let me put it this way... I have a glass of water.. and a photon enters it from the left side..and it is absorbed and is reemiited by the first atom in water.. and the process continues until it comes out from the right side of the water column.. why should this happen? And what is the problem with a piece of wood? why can't the atoms in it do the same thing?
  7. Dec 7, 2009 #6
    Re: Transparency!!

    because of the energy-dependence of the absorption. Different material absorb at different wave-lengths.

    and you should not confuse a photon with a light beam...
  8. Dec 7, 2009 #7
    Re: Transparency!!

    how is a light beam different from a group of photons??? even a peice of wood takes a photon and reflects in back.. but doesn't transmit it.. why is that so? reflection also involves absorbtion and reemission right? but only in the same direction... why is that so?
  9. Dec 7, 2009 #8
    Re: Transparency!!

    you were saying "the photon".....

    as for the wood and reflection, reflection is wavelength dependent as-well... that is why different objects have different colors.
  10. Dec 7, 2009 #9


    User Avatar

    Re: Transparency!!

    I think the problem here is that you don't know/ understand that visible light is just a small part of the Electro Magnetic spectrum. Different frequency photons within the visible part have different colors. Frequencies outside the visible part just cant bee seen with our eyes. All the radio waves, microwaves, infra-red, visible, ultra-violet, x-ray and gamma rays are EM radiation with different frequencies .
    Reflection and transparency is frequency-dependent. Different materials, different frequency.
    Now, glass may be transparent to the small visible part of the spectrum, but at the same time may be reflective or absorptive to, say, infra-red frequencies. A piece of wood may be reflective/ absorptive to visible part, but transparent to radio waves or... x-rays, i am not sure.
  11. Dec 7, 2009 #10
    Re: Transparency!!

    ... and also that these processes are probabilistic
  12. Dec 7, 2009 #11
    Re: Transparency!!

    Do you know about Snell's Law?


    It might also be an idea to read the FAQ here?


  13. Dec 7, 2009 #12
    Re: Transparency!!

    Thanks for this post!!

    I think 50% of my question has been resolved. However, I have this question now.

    Howcome an x-ray photon can pass through the human flesh without any issue but a visible light,say blue photon is reflected back?
  14. Dec 7, 2009 #13
    Re: Transparency!!

    Have you heard of statistical waves.

    Ie group and phase velocity and ?

    Cheronkov radiation.


    Nothing can exceed c not even gamma rays or photons which are made up of them.
  15. Dec 7, 2009 #14
    Re: Transparency!!

    Because X-ray photons have a very large momentum, in comparison to the visible light ones. It's as if you shooted ping-pong balls against concrete (visible light photons) or lead bullets against butter (X-ray photons).
    Furthermore, visible light finds easily chemicals which absorbs it, in the human flesh (actually, in the skin already) because there are many atoms and compounds with electronics energy levels in that range of EM radiation, while for X-rays that's much less probable.
  16. Dec 7, 2009 #15
    Re: Transparency!!

  17. Dec 7, 2009 #16
    Re: Transparency!!

    I didn't really understand the bullet anology... How come a blue photon cannot pass through a less denser milk but can pass through a more dense glass??
  18. Dec 7, 2009 #17


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    Re: Transparency!!

    Because it matters what the material is made of... (?)
  19. Dec 7, 2009 #18
    Re: Transparency!!


    Superposition is clear only when you consider particle/wave duality otherwise it makes no sense.

    If a tree falls in the forest and no one is around to hear it does it make a sound?

    Thus define hear and sound?

    Are they different or not?

    Is a qualia an experience that you can sum to maths?

    Is red for me the same as for you?
  20. Dec 7, 2009 #19


    User Avatar

    Re: Transparency!!

    Now THAT is a freekin good question...
  21. Dec 8, 2009 #20
    Re: Transparency!!

    Because of what I wrote in the second part of my post...
    When you treat light-matter interaction you have to specify what kind of interaction you want to analyze, or the answers would be quite generic. For a photon in the visible range interacting with milk or glass, the main interaction is with the electronic energy levels of the atoms and/or with the phonons of the entire structure (as in the case of glass), so in this case it's not the photon momentum that counts, but its frequency, how that matches those energy levels. If instead the interaction is mainly due to photon/electron scattering (e.g. Compton scattering), then what counts is the photon's momentum.
    Last edited: Dec 8, 2009
  22. Dec 8, 2009 #21
    Re: Transparency!!

    I think lightarrow may be wrong about matching energy levels with phonons. I'm pretty sure photons interact with lattice vibrations ("phonons" as people call them) when the momentum matches up, not when the energy matches. In that respect it's similar to Compton scattering, and for similar reasons. When the momentum matches, the wavelength matches. And for a lattice vibration, if there is any electric polarization associated with the mechanical deformations of the material, you create planes of charge at precisely the wavelength of the incident light, give the light wave something to "bite" into. Hence the interaction.
  23. Dec 8, 2009 #22
    Re: Transparency!!

    They are found in the direction where the wave-interference is constructive.
    Milk is a bad example: Its colour and opacity has to do with larger scale structure and internal reflection rather than intrinsic absorptivity, like comparing clouds to rainwater (or dry paper to oily paper).
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