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Comment by dvh | Hacker News Reader

Metal will never be transparent. Metal means metal bonds which means free electrons. Free electrons are opaque to visible light.

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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.

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)

> you are stating somehow the direction of photon is preserved when absorbed by electron

Well, that is how lasers work :)

https://en.wikipedia.org/wiki/Stimulated_emission

I barely understand even layman-level physics, but it would seem to me that this “absurd” idea is just a natural implication of the conservation of momentum?

edit: to all the downvoters of saiya-jin — let the one who has never defended their incorrect intuitions in physics cast the first downvote!

> let the one who has never defended their incorrect intuitions in physics cast the first downvote!

Amen. This stuff is wildly counter-intuitive, we only properly know how mirrors work since we understand the photo electric effect, and even with that understanding it is still quite tricky because it requires insight into how stuff works at a level where direct observation is no longer possible without access to enormous resources.

I've had that same question before.

This is in fact how it works - otherwise mirrors wouldn't work!

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.

A minute current is induced which then results in the creation of a new photon identical to the original.

They are identical in terms of their quantum mechanical properties.

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.

Ashcroft and Mermin is only an 'introduction' in the graduate student sense of 'introduction'. I don't think you can make sense of it without previous grounding in both quantum mechanics and solid state physics.

Well, he did ask about band gaps. Is there a way to understand band gaps without quantum mechanics? I think you only need a little Fourier analysis to get the basics of band gaps.

Anyway, that text and along with Kittel's are the references for an undergraduate solid state course that I'm taking. No prior exposure to solid state physics for me and only introductory quantum mechanics (first half of Griffiths' QM); I find the text totally approachable.

Semiconductor Device Fundamentals by Robert F. Pierret was my go-to for undergraduate text. The diagrams speak a thousand words and Pierret is a funny dude. Builds everything up from basic quantum.

Don't mistake it with his graduate text, Advanced Semiconductor Fundamentals, though. That's also a great text, but very short and focuses almost exclusively on the quantum aspect without getting too much into the higher level meat of putting it together to form devices.

For a comprehensive guide, though, Physics of Semiconductor Devices by Simon M. Sze was my reference bible. It's big and bulky, very heavy on the first principles math and physics, and has everything from quantum to devices and variants on devices.

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?

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.

My physics is only high school level so as I see, in the case of fixed - does the photon not get scattered/reflected or does it pass through the electron? And why no orbital transition?

Reflection on the surface of polished metals occurs because of the collective movement of electrons at the surface, caused by the incoming photons. Light of most wavelengths, especially low-energetic infrared doesn't have enough energy to cause orbital transitions in most elements.

What about partial ? or only to some wavelengths ?

X-rays penetrate metal to some extent, though they also scatter off the atoms. Radiography of high value/risk metal components (such as the stressed parts of gas turbine engines) is a mainstream non-destructive testing technique.

huh, is this why sometimes metal gets unbelievably hot if left in the sun? but that should be true of any opaque material

For a given temperature rise, metals feel hotter than many other materials because they have a high thermal conductivity and enough heat capacity to deliver a lot of heat quickly.

As another though experiment, consider that you dread walking barefoot across cold tile floors but can bear to walk across carpeted floors in the same house. These two materials are at the same temperature.

Also consider aluminum foil you just pulled out of the oven. It's thinness runs contrary to the large thermal capacity of a solid chunk of metal- you can touch it immediately because you are such a large heat sink compared to it that it can't burn you even while it has only just started (rapidly) cooling from 350°F.

A conductive surface will reflect an EM wave because a lot of math but it can be proven

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.]

Thank you. That's very helpful. I studied the quantized transfer but never bothered to ask what happens if electron is hit by the energy isn't exactly what's required for an orbital transition.

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