Hey, people !

• Why do you think glass is transparent, and aluminum opaque ?
• Would you like to see the same glass suddenly becoming opaque ?.....Well, it can !!

We all know that every material is made up of electrons, protons and neutrons.
The science fellows here will also know that electrons revolve around the nucleus(made up of neutrons and protons - the heavyweight parents of the lean electrons) of an atom at specific distances(often called bands), highly energetic and rioting ones farther from the nucleus in the outer bands, sleepy and docile ones closer to the nucleus in the inner bands !
The outermost electron-containing band in an atom is called the valence band, and contains the most energetic electrons of any stable, neutral atom.
When light falls on an atom, the photons simply collide with the orbiting valence electrons, thus exciting them, making them agile !! These agile electrons being more energetic than the rest in the band, no longer remain in a mood to stay indoors and tend to jump farther away from the nucleus.
But, unfortunately electrons are very choosy as far as their position from the nucleus(their parents for which they have affinity) is concerned. They just don't like staying anywhere and everywhere - on the street, down the river, on their own TV dish antenna ! They'll either go to the grocer's or be in their garden, or go to the gym ! But, they definitely don't like hanging about "in the middle of nowhere" so to say, like any other obedient kid!

So, if Mr.Photon wants his friend electron to go from home(let this be Band-1) to the garden(let this be the next level i.e. Band-2), the photon must provide the electron sufficient amount of agitation, so that the electron can jump to the next level. If the photon fails to deliver sufficient agitation, then the electron would pretend as if nothing actually happened rather than going out and then getting caught by their parents hanging in the middle of the street for no reason !
So, a photon is nothing but a packet of energy or agitation. If the photon collides with an electron and is able to excite the electron, then basically the photon gets absorbed by the electron, else, it simply passes unaffected !

[©http://abyss.uoregon.edu/~js/glossary/wave_particle.html]

So, in the case of glass, as you might have guessed by now, the valence electrons are not in a position to absorb the small amount of energy contained by the photons of visible light. So, these photons entering a glass slab from one side, simply re-emerge unaltered from the other side. This gives an illusion of transparency to the human eye.

But, the same glass will become opaque if the photons incident on it contain sufficient energy so as to be able to excite the electrons to their next energy state. Try shining UV light(more specifically UV-B) on glass and see !

For more curious people, the video by Veritasium below should be great !!

[ ©Veritasium (https://www.youtube.com/channel/UCHnyfMqiRRG1u-2MsSQLbXA)]

Thanks
Keep learning !
M.Medro