Stained Glass Problem: Why Are Low Energy Wavelengths Absorbed?

[Daniel Petka]

Glass is transparent because the light doesn't have enough energy to get the electrons to the next energy level. But how does blue stained glass work? Does the blue light have too much energy to excite the electron in comparison to red and green that get absorbed? Why are the low energy wavelengths being absorbed? Is there another principle?

 

[scottdave]

This is actually a difficult one to find an answer. Starting with how glass is transparent (see the video)
If the photon does not excite the electron, then it passes through. It briefly talks about how some materials may absorb some energy levels (wavelengths) but not others. So some colors would pass through unabsorbed. There is another video (I will try to find it) which shows how certain substance glow under ultraviolet light. So what is happening there, the ultraviolet light (which we cannot see) excites the electron by more than one level, but then it goes back down to an intermediate level (emitting visible light) before returning to ground state.

I am assuming that red light excites it more than one level, then it falls back down and the intermediate step is infrared.

 

[Daniel Petka]

Thanks

 

[Daniel Petka]

Btw the vid doesn't explain why blue photons pass through blue stained glass and red photons don't.

 

[phinds]

Btw the vid doesn't explain why blue photons pass through blue stained glass and red photons don't.

Of course it does, just using a different color filter. Blue photons don't have enough energy to take the electrons in blue glass to the next level, so they are not absorbed. Red photons do have enough energy to do that and so are absorbed.

 

[vanhees71]

Despite the fact that photons don't have a color (but rather affect a color sensation in a human eye), "blue" photons are more energetic than "red" ones ;-).

 

[scottdave]

It is not like "photons above a certain energy are absorbed because they have enough energy", it is more like when he was talking about light not going through a wall, I think. If the red light is energetic enough to excite some "easy" electrons up two levels, for example. But then maybe on the way back "down" they stop at the intermediate level (emitting an infrared level photon). Then they go back to ground state and maybe another infrared photon.

I am not sure if this is exactly how it works or not, but that is what I am thinking. It seems hard to find any information detailing "why" certain atoms (or crystal structures) absorb 1 "color" and not another, but just remember that if the electron is excited, it will eventually go back to ground state, emitting a photon, but not necessarily in the same direction.
Also, it is common to excite more than one level then go back down in intermediate levels. This is how a black light (high energy UV photons) makes certain materials appear to glow. We cannot see the UV rays, but we see the lower energy photons which are emitted by the intermediate stages.

 

[Daniel Petka]

Of course it does, just using a different color filter. Blue photons don't have enough energy to take the electrons in blue glass to the next level, so they are not absorbed. Red photons do have enough energy to do that and so are absorbed.

What?? I thought blue photons have MORE energy than red photons.

 

[phinds]

What?? I thought blue photons have MORE energy than red photons.

Damn. You're right of course. I clearly wasn't paying attention to what I was saying.

 

[Andy Resnick]

Glass is transparent because the light doesn't have enough energy to get the electrons to the next energy level. But how does blue stained glass work? Does the blue light have too much energy to excite the electron in comparison to red and green that get absorbed? Why are the low energy wavelengths being absorbed? Is there another principle?

Blue glass is typically made by adding Cobalt Oxide, which absorbs in the red:

http://pubs.acs.org/doi/pdf/10.1021/ja01330a010

Hence, blue light passes through.

Red glass is tricky- that uses Gold, but the mechanism is not absorption/transmission exactly (AFAIK)- it's more complicated:

http://www.nature.com/nature/journal/v407/n6805/full/407691a0.html