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This question already has an answer here:

What is the difference between a window and a wall that makes a wall opaque but a window transparent, in terms of the molecules that make them up?

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marked as duplicate by John Rennie, ACuriousMind, Kyle Kanos, John Duffield, Sebastian Riese Feb 24 '16 at 15:46

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    $\begingroup$ Hi Sekots, we encourage site members to do at least the obvious search before posting a question. $\endgroup$ – John Rennie Feb 24 '16 at 12:31
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    $\begingroup$ Another question to consider is why can you see through a pane of glass, which is made by melting silicon dioxide (sand), but you cannot see through a pile of sand. They are made of the same substance after all. This shows that material properties alone are not the answer, but also their organization. Some materials in a wall are optical absorbers, while others, like the grains of sand, are optical scatterers. Even glass is an optical absorber, and if thick enough you cannot see through it. $\endgroup$ – Peter Diehr Feb 24 '16 at 12:34
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If you imagine the electron clouds of the materials' atoms to oscillate in reaction to traversing electromagnetic waves, there is - depending on the material - a certain frequency range where resonance occurs (imagine the vibrating clouds to behave like a spring...). If resonance occurs, the material is therefore able to dissipate the electromagnetic energy of the incoming electromagnetic wave, i.e. the electromagnetic wave is absorbed. The resonance frequency of glass windows is in the UV regime, not in the visible range. If your eyes would be sensitive only to electromagnetic radiation in the UV regime, a glass window would look solid.

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  • $\begingroup$ Also, if reflection takes place (i.e. the cloud oscillates but emits the ray back in the side of the incident ray) the glass would look solid as well. $\endgroup$ – Sidarth Feb 24 '16 at 12:50
  • $\begingroup$ @Sidarth: One could easily write a book about the subject. I tried to keep it simple, and therefore, I merely explained the most important (and intuitive) aspect of the phenomenon ;). Feel free to add more details below :p. $\endgroup$ – sperber Feb 24 '16 at 12:52

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