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I just watched a video from the channel "Sixty Symbols" called "Why is glass transparent?". Link: https://www.youtube.com/watch?v=Omr0JNyDBI0

The explanation about why some materials absorb light and some don't was something like this: "When a photon passes through an electron, it elevates the energy level of this electron from the ground level to the upper level and because of that the electron is being absorbed, but in glass this energy gap is too large, so the photon just passes through the atoms."

So, do I understand correctly, that if the photon hit the wall it has to be absorbed. If all the photons are being absorbed by the wall, then how do they bounce back into my eye? As far as I know objects reflect the light that they can't absorb. But let's say I have a red lamp, I shine with a flashlight on it, the red light from the flashlight is not being absorbed by the lamp's surface, but why does it bounces back to me and not just pass through the lamp?

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It is a very bad explanation of what is going on with light and solid matter.

Simply a photon - atom ( or molecule or lattice) interaction is as follows, as a quantum mechanical particle, it has a probability to:

  1. scatter elastically

  2. raise the atom (molecule or lattice) to a higher energy level, and then a new photon will come out , the original either being completely absorbed or a new photon scatters off.

In the case of transparent matter it is the photon-lattice scattering which needs to be understood . The lattice has a quantum state of its own. The photon can scatter elastically with the whole lattice, and also in the forward direction. This means images do not change color and there is a probability of backward scattering for reflections. If the image changes color it means an inelastic scatter with the whole lattice , a new photon with lower energy coming out in the forward direction.

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The last part of your question is not clear to me. What I can get is as below:- Glass has large inter atomic spaces and photons pass through those inter atomic spaces and glass is transparent. And to the question that why you can see the wall, all the photos don't get absorbed by the wall. Wall reflects the photons of its own colour due to which you can see the wall. And you can see the lamp red because it don't have too large interatomic spaces and reflects red light back. So you can see the lamp red.

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    $\begingroup$ This is incorrect. Spacing between atoms in glass is not especially large and atomic spacing is not a reason for transparency in solids and liquids. $\endgroup$
    – g s
    Dec 29, 2022 at 6:39

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