Home Discussion Forum I never questioned it but now... quantum theory and light "photons"?

I never questioned it but now… quantum theory and light "photons"?

When light behaves like particles, do they suffer any resistance on crossing thorough things like glass?


  1. There is not a simple answer, but there is an answer. But I am not Richard Feynman! And even he never claimed to really understand QM!
    First, some light is always ‘reflected’ – or is emitted ‘backwards’ – at the boundary to a denser medium: both as it enters and as it leaves.
    Second, the photons are repeatedly absorbed by orbital electrons of the denser material, then the energy is quickly emitted by these same electrons as ‘new’ photons. While the energy of the photon is held by the electorn, the photon does not exist, and so cannot be moving at the speed of light. This process is what causes the speed of light in a medium to be less than in a vacuum.
    However! And this is a BIG “however”, no energy is lost in this process. There is no automatic production of heat.
    Of course, I am speaking about a theoretical medium. Real mediums are not perfectly transparent, and so you do have some energy absorbed and radiated as heat.
    But from the quantum perspective, the absorption of energy by an electron and also the emission of that energy, take place instantly and with 100% efficiency.
    So you have a situation that cannot occur in the non-quantum world: in the macro world, resistance always implies energy lost to heat. In QM you have interaction, but no net loss of energy. I would call that ‘no resistance’.

  2. It depends on the state of the medium. Photons can be absorbed leading to the medium becoming hotter, just as electrons can lose energy through collisions with a lattice of atoms causing a wire to heat up.
    However, in some cases you can get more light out than you put in. Lasers work by exciting atoms and then having them release their excitation energy when they are hit by another photon. This causes amplification of the light. The atoms may become excited through chemical reactions and the laser then turns chemical energy into light.


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