5 ms·

Hmm, so by adding extra electrons (and ions?) to water, the water gains reflective properties. From a bit of googling, I'm reading that reflectiveness comes fr

Hmm, so by adding extra electrons (and ions?) to water, the water gains reflective properties. From a bit of googling, I'm reading that reflectiveness comes from having small "band gaps" (gaps between allowed electron energy levels) for electrons to cross. Metals have small/no band gaps, while non-metals have larger ones that (if I understand correctly) don't match with the energy levels of visible light. So I assume this means that adding electrons to water changes its band structure and reduces the gap. So is it generally true that any non-metal which gains electrons is able to absorb and release visible-wavelength photons more easily, i.e. is reflective?

CamperBob2 5y ago
So is it generally true that any non-metal which gains electrons is able to absorb and release visible-wavelength photons more easily, i.e. is reflective? Bill Beaty has a nice ELI5-style essay on the subject at http://amasci.com/elect/charge1.html http://amasci.com/elect/charge1.html , where he discusses what electric charge is (and isn't): "Here is a way to see charge directly: look at the surface of a wire. Metals look metallic because they contain a 'fluid' composed of movable electrons. This electrical 'fluid' is an excellent reflector of light waves, and it causes the surfaces of metals to act like mirrors. It's these same electrons which flow during an electric current. The 'silvery' stuff of a metal is the charge. What is charge? It is a 'silver liquid' which is found in all metals, and which can be forced to flow. "Even though the charge is visible, its flow is not. Look carefully at wires in an operating electric circuit and you won't see anything moving along. This is not very mysterious: stir a glass of water and then look for the flowing motion. You'll see moving bubbles and perhaps moving specks of dirt, but you won't see the water move. The silvery charge-fluid in a wire has no bubbles or dirt, so even though the charge is visible, we cannot tell if it is moving or still."
- creamyhorror 5y ago
  > This electrical 'fluid' is an excellent reflector of light waves Thanks, though I'm actually wondering about a deeper question: why is this so? Why do loose plasma-like electrons make metals reflective (or, roughly equivalently, how does adding extra electrons make a non-metal like water more reflective)? To continue on this topic, the 'bands' I referred to are clusters of energy levels corresponding to the orbitals of the atoms. (I found this neat explanation: https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Map%3A_Inorganic_Chemistry_(Housecroft)/06%3A_Structures_and_energetics_of_metallic_and_ionic_solids/6.08%3A_Bonding_in_Metals_and_Semicondoctors/6.8B%3A_Band_Theory_of_Metals_and_Insulators https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/... ). Apparently, metallic reflectivity comes down to the fact that metals have overlapping "valence" and "conduction" bands. This allows electrons in the lower "valence" band to be excited to the higher "conduction" band and then fall back down and emit photons - which is reflection. Therefore, in the case of water with extra electrons, the additional electrons may be causing the valence and conduction bands to come closer together - maybe by filling up the valence band and bringing the top of the band closer to the conduction band? Just wanted to speculate on the chemistry of this discovery a bit, I've always found the science interesting.
  - pas 5y ago
    Because the electrons in the shared "metallic" electron cloud are in a myriad of quantum states, thus they are able to interact with myriad of "kind" of photons. (Of course the important thing is the wavelength = frequency = energy of the photon.) In particular for metals (like mirrors) where there's a lot of "free" electrons this interaction is through the EM field of all those electrons, not through "isolated" photon-atom pairs. The incoming photons are themselves traveling excitations of the EM field. The electrons - as charged particles - are nudged by the EM field. The moving charges then cause new EM waves, the outgoing photons. In transparent stuff the electrons are bound to nuclei, and almost all of the photons in the human visible energy range just go through without interacting with the electrons. ( https://www.lehigh.edu/imi/teched/GlassProp/Slides/GlassProp_Lecture18_Lucas3.pdf https://www.lehigh.edu/imi/teched/GlassProp/Slides/GlassProp... ) In opaque stuff, let's say a black cloth, the bound electrons are able to absorb almost all of visible light and then emit it as lower frequency (infrared) photons. In mirrors and shiny stuff, the electrons are able to scatter most of the photons. (So the conversion to infrared photons - heat - is usually negligible.) See this for more details: https://www.reddit.com/r/askscience/comments/1zi5hg/how_do_atoms_reflect_light_perfectly_mirror/cftxnsd/ https://www.reddit.com/r/askscience/comments/1zi5hg/how_do_a...
    - creamyhorror 5y ago
      Quite interesting - the details get ever more complex as you dig deeper. So the incoming photons interact with the oscillations of the free electrons (i.e. "plasmons"), causing new EM waves to be emitted. I take it that this interaction between photons and plasmons is equivalent to the changing of energy states of electrons and the release of photons as electrons fall to lower states. So in non-metals, does the band gap between the valence and conduction bands get crossed by electrons excited by incoming photons? You said a black cloth is able to absorb and emit virtually all visible light, but as infrared, but how do band gaps work there? I guess this is too advanced for a casual conversation with a layman like me, so thanks for the explanations. In the end, maybe it just boils down to "free electrons reflect light because that is what they do".
      - 5y ago

CamperBob2 5y ago

So is it generally true that any non-metal which gains electrons is able to absorb and release visible-wavelength photons more easily, i.e. is reflective? Bill Beaty has a nice ELI5-style essay on the subject at http://amasci.com/elect/charge1.html http://amasci.com/elect/charge1.html , where he discusses what electric charge is (and isn't): "Here is a way to see charge directly: look at the surface of a wire. Metals look metallic because they contain a 'fluid' composed of movable electrons. This electrical 'fluid' is an excellent reflector of light waves, and it causes the surfaces of metals to act like mirrors. It's these same electrons which flow during an electric current. The 'silvery' stuff of a metal is the charge. What is charge? It is a 'silver liquid' which is found in all metals, and which can be forced to flow. "Even though the charge is visible, its flow is not. Look carefully at wires in an operating electric circuit and you won't see anything moving along. This is not very mysterious: stir a glass of water and then look for the flowing motion. You'll see moving bubbles and perhaps moving specks of dirt, but you won't see the water move. The silvery charge-fluid in a wire has no bubbles or dirt, so even though the charge is visible, we cannot tell if it is moving or still."

creamyhorror 5y ago
> This electrical 'fluid' is an excellent reflector of light waves Thanks, though I'm actually wondering about a deeper question: why is this so? Why do loose plasma-like electrons make metals reflective (or, roughly equivalently, how does adding extra electrons make a non-metal like water more reflective)? To continue on this topic, the 'bands' I referred to are clusters of energy levels corresponding to the orbitals of the atoms. (I found this neat explanation: https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Map%3A_Inorganic_Chemistry_(Housecroft)/06%3A_Structures_and_energetics_of_metallic_and_ionic_solids/6.08%3A_Bonding_in_Metals_and_Semicondoctors/6.8B%3A_Band_Theory_of_Metals_and_Insulators https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/... ). Apparently, metallic reflectivity comes down to the fact that metals have overlapping "valence" and "conduction" bands. This allows electrons in the lower "valence" band to be excited to the higher "conduction" band and then fall back down and emit photons - which is reflection. Therefore, in the case of water with extra electrons, the additional electrons may be causing the valence and conduction bands to come closer together - maybe by filling up the valence band and bringing the top of the band closer to the conduction band? Just wanted to speculate on the chemistry of this discovery a bit, I've always found the science interesting.
- pas 5y ago
  Because the electrons in the shared "metallic" electron cloud are in a myriad of quantum states, thus they are able to interact with myriad of "kind" of photons. (Of course the important thing is the wavelength = frequency = energy of the photon.) In particular for metals (like mirrors) where there's a lot of "free" electrons this interaction is through the EM field of all those electrons, not through "isolated" photon-atom pairs. The incoming photons are themselves traveling excitations of the EM field. The electrons - as charged particles - are nudged by the EM field. The moving charges then cause new EM waves, the outgoing photons. In transparent stuff the electrons are bound to nuclei, and almost all of the photons in the human visible energy range just go through without interacting with the electrons. ( https://www.lehigh.edu/imi/teched/GlassProp/Slides/GlassProp_Lecture18_Lucas3.pdf https://www.lehigh.edu/imi/teched/GlassProp/Slides/GlassProp... ) In opaque stuff, let's say a black cloth, the bound electrons are able to absorb almost all of visible light and then emit it as lower frequency (infrared) photons. In mirrors and shiny stuff, the electrons are able to scatter most of the photons. (So the conversion to infrared photons - heat - is usually negligible.) See this for more details: https://www.reddit.com/r/askscience/comments/1zi5hg/how_do_atoms_reflect_light_perfectly_mirror/cftxnsd/ https://www.reddit.com/r/askscience/comments/1zi5hg/how_do_a...
  - creamyhorror 5y ago
    Quite interesting - the details get ever more complex as you dig deeper. So the incoming photons interact with the oscillations of the free electrons (i.e. "plasmons"), causing new EM waves to be emitted. I take it that this interaction between photons and plasmons is equivalent to the changing of energy states of electrons and the release of photons as electrons fall to lower states. So in non-metals, does the band gap between the valence and conduction bands get crossed by electrons excited by incoming photons? You said a black cloth is able to absorb and emit virtually all visible light, but as infrared, but how do band gaps work there? I guess this is too advanced for a casual conversation with a layman like me, so thanks for the explanations. In the end, maybe it just boils down to "free electrons reflect light because that is what they do".
    - 5y ago