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Comment by dbsvsv | Hacker News Reader

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?

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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.

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

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