Metals don't reflect UV?

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ASGtFT
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Metals don't reflect UV?

Postby ASGtFT » Wed Jun 01, 2011 10:54 am UTC

Hey,

I heard somewhere that metals are transparent under UV light. Can anyone *ahem* shed some light as to why that is?

eigengirl
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Re: Metals don't reflect UV?

Postby eigengirl » Wed Jun 01, 2011 11:02 am UTC

Shining light onto a metal causes the electrons in the metal to oscillate according to the classical dipole oscialltor model - essentially there is a driving force (the electric field of the incident radiation), a damping force and a resonant oscillation. We can calculate the dielectric permittivity from this model, and from this we can also calculate the reflectivity, which characterises which wavelengths of light are reflected, transmitted, and absorbed.
According to this model (it's called the Drude-Lorentz model), the transition metals (for example, copper, gold) should reflect light at UV wavelengths. Yet they appear gold/yellow to us.
Clearly there must be some kind of absorption mechanism that has not been accounted for in the model we used to make the prediction. It turns out that there are allowed transitions between two bands (energies that are allowed) of electrons (for example, in copper it's the 3d and 4s), which means that incident light which has an energy resonant with this gap will be absorbed. The energy gap between these two bands is about 2eV, which corresponds to UV wavelengths. So that's why UV light is not reflected.

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Re: Metals don't reflect UV?

Postby Aelfyre » Wed Jun 01, 2011 6:28 pm UTC

eigengirl wrote:Shining light onto a metal causes the electrons in the metal to oscillate according to the classical dipole oscialltor model - essentially there is a driving force (the electric field of the incident radiation), a damping force and a resonant oscillation. We can calculate the dielectric permittivity from this model, and from this we can also calculate the reflectivity, which characterises which wavelengths of light are reflected, transmitted, and absorbed.
According to this model (it's called the Drude-Lorentz model), the transition metals (for example, copper, gold) should reflect light at UV wavelengths. Yet they appear gold/yellow to us.
Clearly there must be some kind of absorption mechanism that has not been accounted for in the model we used to make the prediction. It turns out that there are allowed transitions between two bands (energies that are allowed) of electrons (for example, in copper it's the 3d and 4s), which means that incident light which has an energy resonant with this gap will be absorbed. The energy gap between these two bands is about 2eV, which corresponds to UV wavelengths. So that's why UV light is not reflected.



Ok so I am thinking that not reflecting UV light and being *transparent* to it are two separate phenomena..

Surely the latter isn't the case.. otherwise you could see through the walls at most nightclubs.. LOL well maybe not.. but still I just can't see UV light penetrating any normal metal as if it were glass.. or at least I've never heard of the phenomena
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Re: Metals don't reflect UV?

Postby Moose Hole » Wed Jun 01, 2011 6:52 pm UTC

Of course metal is transparent to UV. Why do you think medieval knights had such great tans?

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Kang
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Re: Metals don't reflect UV?

Postby Kang » Wed Jun 01, 2011 7:13 pm UTC

ASGtFT wrote:Hey,

I heard somewhere that metals are transparent under UV light. Can anyone *ahem* shed some light as to why that is?

Could it be that you've heard such in the context of anybody trying to make quantum mechanics 'easily' understandable? When taking a historic approach to that it likely refers to blackbody radiation, the point being that the radiation maximum is temperature-dependant (see Wien's displacement law). Arguably the colour in which hot metal just taken out of the furnace differs from red to yellow depending on temperature, but I admit never having checked on whether that really is the decisive effect here, it's being used in explanation attempts nonetheless. Following Wien's displacement law further a steel bar you keep heating would change from orange to green, then to blue and eventually become increasingly dark when the maximum wavelength moves into UV. At least one documentary I once saw got that wrong by claiming that it would become transparent.

P.S.:
Of course only the maximum wavelength leaves the visual spectrum eventually, so it would still follow a Planck distribution, not become pitch black, but you get the idea.

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Re: Metals don't reflect UV?

Postby Game_boy » Wed Jun 01, 2011 8:16 pm UTC

Kang wrote: Following Wien's displacement law further a steel bar you keep heating would change from orange to green, then to blue and eventually become increasingly dark when the maximum wavelength moves into UV. At least one documentary I once saw got that wrong by claiming that it would become transparent.


Does this happen? I'd love to see a video of a metal bar glowing green or blue. Does it just melt before that?
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Re: Metals don't reflect UV?

Postby Izawwlgood » Wed Jun 01, 2011 8:35 pm UTC

For what it's worth, Diamonds are transparent to UV.
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Re: Metals don't reflect UV?

Postby LaserGuy » Wed Jun 01, 2011 8:41 pm UTC

Game_boy wrote:
Kang wrote: Following Wien's displacement law further a steel bar you keep heating would change from orange to green, then to blue and eventually become increasingly dark when the maximum wavelength moves into UV. At least one documentary I once saw got that wrong by claiming that it would become transparent.


Does this happen? I'd love to see a video of a metal bar glowing green or blue. Does it just melt before that?


It may depend on the metal, but I believe a lot of metals will melt well before this point. If you've ever tried blacksmithing, you'll find that metals typically go red -> orange -> yellow -> white -> melting or sparking. The metal looks white because the blackbody spectrum is centred around the green or blue, but it's sufficiently broad that you see colours out to the red, and the intensity across all colours is sufficiently high that your eye is basically saturated. I think that the temperature would have to be very hot indeed before it would start to look dark to your eye, because the tail on the blackbody spectrum is very long, so you'd always expect some radiation in the visible. Probably the metal would turn to vapour before this point, I would imagine.

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Re: Metals don't reflect UV?

Postby dainbramage » Wed Jun 01, 2011 9:11 pm UTC

The metal should never 'go dark' with increasing temperature. The peak moves into UV, but the radiation at all frequencies increases with temperature.

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Re: Metals don't reflect UV?

Postby Kang » Wed Jun 01, 2011 9:37 pm UTC

dainbramage wrote:The metal should never 'go dark' with increasing temperature. The peak moves into UV, but the radiation at all frequencies increases with temperature.

True, that argument worked under the simple assumption that one only looked at the maximum intensity wavelength. One could say that in theory, if one had a detector that can't be saturated one would see the colour change through the optical at least, though.

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Re: Metals don't reflect UV?

Postby Tass » Thu Jun 02, 2011 6:32 am UTC

Kang wrote:Following Wien's displacement law further a steel bar you keep heating would change from orange to green, then to blue and eventually become increasingly dark when the maximum wavelength moves into UV. At least one documentary I once saw got that wrong by claiming that it would become transparent.


dainbramage wrote:The metal should never 'go dark' with increasing temperature. The peak moves into UV, but the radiation at all frequencies increases with temperature.


Neither does anything ever glow green purely from heat. (If it has specific transitions it is a different story). It drifts from yellow into white and then finally a bluish white at very high temperatures. At this point the relative amount of visual wavelengths are constant and the only increase is in the UV, which doesn't change appearance as it is invisible.

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Re: Metals don't reflect UV?

Postby p1t1o » Thu Jun 02, 2011 10:24 am UTC

You can however, make a guess at the temperature on a piece of hot steel based on its colour before it becomes incandescent due to different types of oxide that form at different temperatures, I have seen a piece of film once showing a quite strikingly blue piece of steel, this equates to somewhere around 300C.

Looks like the difference between reflection, absorption and transparency has been covered, so my contribution is: something that seems shiny isn't necessarily a great reflector, for example, a piece of white paper reflects a lot more incident light than a standard mirror does.

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Re: Metals don't reflect UV?

Postby BeerBottle » Thu Jun 02, 2011 4:33 pm UTC

eigengirl wrote:Shining light onto a metal causes the electrons in the metal to oscillate according to the classical dipole oscialltor model - essentially there is a driving force (the electric field of the incident radiation), a damping force and a resonant oscillation. We can calculate the dielectric permittivity from this model, and from this we can also calculate the reflectivity, which characterises which wavelengths of light are reflected, transmitted, and absorbed.
According to this model (it's called the Drude-Lorentz model), the transition metals (for example, copper, gold) should reflect light at UV wavelengths. Yet they appear gold/yellow to us.
Hey eigengirl. Call me an ignorant chemist but I thought that the Drude-Lorentz model did accurately predict that metals are transparent to UV light? So metals reflect light with photon energy below their plasma energy (which is typically around 10 eV, therefore metals reflect photons with wavelengths longer than 125 nm (visible light ranges from 380-750 nm)) You're right that for some metals like gold and copper, there is another contribution from the intra band transitions. But the energy gap of 2 eV for Cu that you quoted is not in the UV but in fact is visible. High absorption in the visible due to this electronic transition is linked to high reflectivity because these photons are re-emitted, which causes Cu and Au to appear coloured.

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Re: Metals don't reflect UV?

Postby idobox » Fri Jun 03, 2011 12:37 pm UTC

I thought that if you exposed metal to UV light, you got photoelectrical effect, with the energy of the photons used to strip and accelerate electrons aways from the metal. Which would be incompatible with re-radiation or transparency.
Maybe it happens only for shorter wavelengths, or only for a limited number of photons?
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Re: Metals don't reflect UV?

Postby p1t1o » Fri Jun 03, 2011 2:31 pm UTC

idobox wrote:I thought that if you exposed metal to UV light, you got photoelectrical effect, with the energy of the photons used to strip and accelerate electrons aways from the metal. Which would be incompatible with re-radiation or transparency.
Maybe it happens only for shorter wavelengths, or only for a limited number of photons?



Indeed the photoelectric effect is significant, but below the minimum frequency required for it (dependant on work function etc. etc.), the above processes occur - and if science has taught me anything, they are likely to continue to occur at reduced proportions even into the photoelectric realm.

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Re: Metals don't reflect UV?

Postby Moose Hole » Fri Jun 03, 2011 4:47 pm UTC

This is all very interesting. How would one modify aluminum so that the energy gap is in the visible range? Or should I just punch up clear?

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Re: Metals don't reflect UV?

Postby Aelfyre » Fri Jun 03, 2011 5:49 pm UTC

Moose Hole wrote:This is all very interesting. How would one modify aluminum so that the energy gap is in the visible range? Or should I just punch up clear?


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Re: Metals don't reflect UV?

Postby p1t1o » Sat Jun 04, 2011 12:43 am UTC

Can't do aluminium, but concrete? Bob's yer uncle!

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Re: Metals don't reflect UV?

Postby Soralin » Sat Jun 04, 2011 1:06 am UTC

You can do it with aluminum too, with some oxygen and nitrogen added: http://en.wikipedia.org/wiki/Aluminium_oxynitride
Aluminium oxynitride or AlON is a transparent polycrystalline ceramic with cubic spinel crystal structure composed of aluminium, oxygen and nitrogen. It is currently marketed under the name ALON by Surmet Corporation.[3] ALON is optically transparent (≥80%) in the near ultra violet, visible and near infrared regions of the electromagnetic spectrum. It is 4 times harder than fused silica glass, 85% as hard as sapphire and nearly 15% harder than that of magnesium aluminate spinel. The material is stable up to 1,200 °C (2,190 °F).[1]

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Charlie!
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Re: Metals don't reflect UV?

Postby Charlie! » Sat Jun 04, 2011 6:49 am UTC

Is it a metal? No? Then not really what's meant.
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Re: Metals don't reflect UV?

Postby idobox » Sat Jun 04, 2011 4:24 pm UTC

Charlie! wrote:Is it a metal? No? Then not really what's meant.

Aluminium will quickly be covered by a layer of opaque oxyde under normal atmosphere. Any treatment you will do to colour it, you will need some sort of coating to keep the color visible, so using a colored coating is not that bad.
Anyway, I don't see how to create a bandgap in a metal that lacks it. By carving nanometric patterns, you could get some kind of irridescence though.
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Re: Metals don't reflect UV?

Postby BeerBottle » Mon Jun 06, 2011 10:18 am UTC

idobox wrote:Anyway, I don't see how to create a bandgap in a metal that lacks it. By carving nanometric patterns, you could get some kind of irridescence though.


It's the plasmon energy not the band gap we need to change. In a metal, light with energy below the plasmon energy is reflected, and with energy above it, transmitted. The plasmon energy depends on the electron density in the conduction band, which we'd need to reduce to move the plasmon energy of Al from the UV to the infrared. Starting from a metal, this is hard to do. Maybe drastically reducing the density would do it, but I'm not sure how to achive that in real life. More success might come starting from a semiconductor and doping it to put only a few electrons in the conduction band, so the plasmon eneny is lower than the visible region. This is the basis for transparent conducting oxides like Indium Tin Oxide.

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Re: Metals don't reflect UV?

Postby idobox » Mon Jun 06, 2011 11:53 am UTC

BeerBottle wrote:The plasmon energy depends on the electron density in the conduction band

Okay, I didn't know that.

Theorically, it's not that difficult to change the elctron density, and it's done commonly with semiconductors, although the initial density is much lower. It might still be doable with thin sheets of metal.
Three ways:
-Build a capacitor with very thin sheets of metal and the best insulator you can find, plug it to a high voltage DC source. Electrons will move from one plate to the other. If the metal is thin enough, you can get a significant depletion before destroying the insulator.
-Heat your metal or expose it to short UV while in a strong electrical field, electrons should be stripped of.
-Connect the metal sheet to highly doped Ptype semiconductor. The electrons of the metal should cancel out with the holes in the semiconductor. The semiconductor will need to be significantly larger than the metal, since its charge carrier density is much lower.
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Re: Metals don't reflect UV?

Postby dainbramage » Thu Jun 09, 2011 9:11 am UTC

This is the sort of territory where metamaterials come into play. A simple example - a wire array can be treated as a metal with a modified plasma frequency with incident TM light. Under TE light it acts as a dielectric though

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Re: Metals don't reflect UV?

Postby sikyon » Fri Jun 10, 2011 3:32 am UTC

idobox wrote:
BeerBottle wrote:The plasmon energy depends on the electron density in the conduction band

Okay, I didn't know that.

Theorically, it's not that difficult to change the elctron density, and it's done commonly with semiconductors, although the initial density is much lower. It might still be doable with thin sheets of metal.
Three ways:
-Build a capacitor with very thin sheets of metal and the best insulator you can find, plug it to a high voltage DC source. Electrons will move from one plate to the other. If the metal is thin enough, you can get a significant depletion before destroying the insulator.
-Heat your metal or expose it to short UV while in a strong electrical field, electrons should be stripped of.
-Connect the metal sheet to highly doped Ptype semiconductor. The electrons of the metal should cancel out with the holes in the semiconductor. The semiconductor will need to be significantly larger than the metal, since its charge carrier density is much lower.


Yeah for macroscopic sizes of metals, the sheer # of electrons is typically too high to create depletion in it.

The first option you suggested is possible, Ie. taking advantage of field emission tips under extremely high bias. However, you will end up with a metal at extremely high bias and you won't be able to really touch it with anything not also under extremely high bias.

The second option is basically creating a plasma, but that is a gas-like state.

The third option will work for extremely (extremely) small pieces of metal. I am not even sure how small it would have to be, but it would literally be tiny, possibly around the 1nm or less range. When electrons and holes deplete at a junction, it creates a reveres bias field that limits the migration of carriers.


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