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Metal will never be transparent. Metal means metal bonds which means free electrons. Free electrons are opaque to visible light.
If the film is thin enough metal is transparent. See for instance 50/50 mirrors and gold plated windows.

Of course you can now argue that if the photons hit the metal they will not pass through, but that's not how it works: the photons will excite an electron to a higher orbit and it may drop back to a lower orbit on the other side of the film making the metal appear transparent or it may reflect.

edit: saiya-jin I can't reply to your comment but yes, the direction is preserved. The same happens with a mirror, the photons ejected will be ejected at the correct angle even though the photoelectric effect has absorbed the photons. That's why metals reflect the way they do!

https://www.scientificamerican.com/article/how-do-mirrors-re...

The small fraction of photons that is absorbed will heat up the mirror.

This is the only relevant part:

>"In a quantum-mechanical picture, light consists of photons, or packages of optical energy. The photons of the light reflected from a metal (or a dielectric mirror) are identical to the incident ones, apart from the changed propagation direction."

This doesn't explain anything about how it works quantum mechanically.

that doesn't make sense - if that would be the case, the back-emitted photon would have a random vector, not pertaining the same one as original one (thus preserving the picture beyond the sheet of metal).

you are stating somehow the direction of photon is preserved when absorbed by electron - absurd idea even for layman physics (not claiming I know how this works, but this can't be the way)

Just to add on for others reading your comment: Although metals are opaque, conductors can still be transparent. A few techniques:

- Very thin layers (like graphene) such that absorption is on the order of a percent

- Narrow bandwidth materials that are opaque to infrared but transparent to visible

- Bad metals and non-band conductors that have correlated electron hopping from site to site

Transparent conductors like ITO are critical materials for displays and solar cells, since light needs to enter/exit one side of the device.

See this experiment for a real transparent metal. Admitedly, the conditions are kinda extreme :)

https://www.youtube.com/watch?v=BIGMfai_ICg

Don't mind the intro, the interesting part starts at around 2:00

I'd never realised the two were connected. What's a good learning reference for bandgap physics? It's something I've never quite understood properly.
You don't need bandgaps to understand why conductors (other than thin films of ITO) don't transmit light. You derive it from Maxwell's equations and the fact that in a conductor, current density = conductivity * electric field. Griffiths' Introduction to Electrodynamics is the standard undergraduate textbook on electromagnetism, and explains this reasonably well. For bandgaps, Ashcroft and Mermin's Introduction to Solid State Physics is what I'm reading from right now, but you don't need it to understand why metals (conductors) don't transmit light.
I'm not so sure about that, ever hard of plasmonics?
I don't think in quite understand. You mean they don't let photons to pass through? In which case do electrons in orbit? Or nuclei for example? Or does this need advanced physics knowledge?
On a quantum level, when a photon encounters a material you have to ask whether the material is able to absorb a photon with that wavelength. When electrons are bound to a specific atom (or are in specific molecular bonds) then there are only certain energy levels possible: that's one of the core features of quantum systems, and it's the reason that each material has its own characteristic "absorption spectrum" (or emission spectrum: same idea). Photons whose wavelength corresponds to an energy that doesn't very closely match what's necessary to raise an electron from one specific level to another will just pass on through. (Nuclei are in bound states with discrete energy levels, too, so they work the same way.)

But one of the essential features of a metal is that the atoms all share a bunch of electrons that are free to move around more or less any way they'd like throughout the material. Because the electrons aren't trapped in one specific bound state, they have an essentially continuous range of energies available to them (just speed up or slow down a little to change your energy), so they are able to absorb photons of any wavelength at all.

[Now, to actually understand why you get reflection rather than stopping with absorption would take me a little more work to figure out how to explain. My instinct keeps being to go back to the classical explanations at that point, but I wanted to focus on quantum here to address your question about electrons in orbit.]

Roughly light is electromagnetic radiation and puts force on electrons it comes in contact with. If the electrons are fixed in a non conductor they don't move much and so don't absorb the energy. If they can move as in most metals the force accelerates them and they absorb energy from the radiation, stoping of reducing it.
A conductive surface will reflect an EM wave because a lot of math but it can be proven
This definitely looks like something that has the potential to be really useful, and gives the example of replacing gorilla glass. While I get that this is a very tough glass, I did not see anything about it's hardness in relation to gorilla glass, as hardness is the property that provides the scratch resistance that is so highly sought after in smartphone screens.

Toughness is how much energy a material can absorb, whereas hardness is the resistance to deformation. Think a rubber band vs. glass.

According to wikipedia's AlON page it has a knoop hardness of ~1800 which from my understanding of things (I may have misread sources or be mistaken, I'm not a material scientist or even amateur) looks to be about equivalent to sapphire glass and much higher than gorilla glass (~600).

edit: in fact while I skipped the intro it states specifically that AlON has ~85% the hardness of sapphire, which more or less checks out. Suffice to say it has excellent hardness, way beyond gorilla glass.

I could find no data on relative permittivity though, and I assume that would be a factor for touchscreens.

I believe hardness is actually a bad thing in terms of replacing gorilla glass in phones. It increases the likelihood of shattering when dropped, and is the main reason for sapphire not being adopted.
Is there are reason why artificial diamond coating hasn't taken off? Cost?
I wish it was possible to buy a phone with the option of a screen who’s glass was optimised for toughness rather than hardness, or at least an aftermarket replacement.

I can live with a screen protector, but broken glass is the bane!

I would assume "rugged(ized)" phone series have this sort of tradeoffs
The deal is that it isn’t aluminium. Seriusly, what’s with the clickbate headline? If you consider everything that contains aluminium atoms in the structure to be actual aluminium, then Saffire is also transparent aluminium.

The authors are diliberatly misleading their readers in order to cincrease interest. That’s a shitty thing to do in science even if do have a cool materiale on It’s own merits.

The headline relates to a line in an old movie (which is mentioned in TFA At the bottom of the page).
To add to this, the "transparent aluminum" is really aluminum oxynitride crystals.
We've updated the title above.
The mental imagery of denting, bending and crushing a transparent glass-like material and having it react the same way aluminium would is immensely satisfying for some reason
Yup, but it won't happen with "transparent aluminum", it behaves more like a ceramic does, so don't expect to indulge your bending and denting urges on it.
Am I the only one to recoil at the thought of someone chewing transparent aluminum foil?

The potential is tremendous in aviation. Imagine transparent aircraft skins -- the superstructure and internals (fuel tanks, hydraulics, etc.) could be inspected without disassembly (which itself adds stress to the structure). Though I'm not sure passengers would take kindly to aircraft with transparent skins. Sometimes it's best that things are hidden under a bonnet.

Other examples of "transparent aluminum": ruby and sapphire, both of them varieties of the mineral corundum, or Al2O3.
Slashdot, 2004:

https://slashdot.org/story/04/08/23/1141217/transparent-alum...

tl;dr: Transparent alumin_a_, not transparent alumin_um_.

On a completely separate note, what was the deal with the need for transparent aluminum in Star Trek IV?

Looking at the footage, they easily could have made the tank bigger if they just used all of the space available and didn't need to be able to "see" them from outside their tank. All they would have needed the aluminum for is to keep water out of where water shouldn't go and regular aluminum (or other material) would have worked just as well.

I don't think they used transparent aluminum in Star Trek IV.

I thought they paid for whatever they used (plexiglass?) with the recipe for transparent aluminum.

Although it could just be that I'm forgiving the poor writing.

It's been a while since I've watched the movie, but I don't think they specifically needed transparent aluminum, but it's how Scotty paid for them getting what they needed. They didn't have money to pay for it, so this was a barter, the knowledge of how to make it for making them some.
Aluminum corrodes a bit in sea water although it didn't need to last that long.
Maybe its stronger than regular
Saying this is transparent Aluminum is the same as saying Glass is transparent Silicon. It's not, it's just a ceramic.
The title is a reference to a line in the movie "Star Trek IV: The Voyage Home" where Scotty trades the formula for "transparent aluminum" (from the future) in a barter transaction for plexiglass sheets (in the present) that the cast need to achieve their goal in the movie.

https://en.wikipedia.org/wiki/Star_Trek_IV:_The_Voyage_Home

https://en.wikipedia.org/wiki/Transparent_aluminum#Transpare...

This isn't new, I've read articles on AlON before. Synthetic Sapphire is more interesting in that its use is growing quickly due to demand for LED and laser substrates. Large single crystals are grown and sliced into wafers: https://en.wikipedia.org/wiki/Kyropoulos_process

New producers of the High purity Alumina (HPA) needed to produce sapphire are coming online, perhaps prices will come down enough that phone screens are an application, but I suspect that this is marketing speculation from the HPA makers to attract investors. For example this presentation from a HPA company speculated in 2015 that the iphone 7 would use a sapphire screen, which turned out to be wrong: https://www.altechchemicals.com/sites/altechchemicals.com/fi...

Does the fact that there is aluminum inside of the ceramic mean that it has a higher conductance? I couldn't find a reference online (there seems to be a few papers over the conductance of two very thin plates of AL203, but no characterization)
While it isn't a metal its still amazing stuff. Sometimes I wish I had stuck with my PhD in Chemistry which would have been working for someone who made equally wild materials.
At last, the whales can be saved!
Synthetic sapphire is a trivial example of a transparent ceramic made with aluminium. Its formula is something very close to Al_2 O_3.
I believe the first Apple Newton had transparent aluminium for the screen. I can't find confirmation of that but I remember going to a sales training session for it (long story) and they told people to say the screen was transparent aluminium, like they talked about in one of Star Trek movies.
This is about the same material which transparent circuit boards are fabricated with: http://www.dk-ceramics.com/transparent-pcb/
Jerry Seinfeld's new material has become super arcane.
I think transparent aluminum foil would be awesome - I could tell if my potatoes are cooked on the BBQ without opening the packet.
tl;dr the substance is probably a ceramic, aluminum oxynitride. Which seems a bit of a cop out like calling regular glass transparent sodium because it contains some. It seems quite cool stuff though.