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Old Jan 25th 2016, 03:47 PM   #1
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Smile How thick does a material have to be to start to become opaque to light?

How thick does a material have to be to start to become opaque to light?
Someone said a atom is transparent to light to me.
but as a material get more thicker, either in single atoms, or a micron, electrons start to absorb light, and there is no more transparency
How thick does a material have to be before there is no more transparency.
Like the material acts as a wall to light.
Now solid state physics can be VERY DIFFERENT than atomic physics.
When atoms are joined together to form a solid, their behavior can be very different than when they are isolated. There are no more "discrete energy states" at the low-lying range, because these have been replaced by energy BANDS!.
If I take carbon atoms, and arrange it in one way, I get graphite.
But I change the arrangement in another way, I get diamond!
Two completely different types of material, and with different optical properties.
Yet, each one of them is made of carbon.
Thank you for your help, anything helps,even a few words.
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Old Jan 26th 2016, 08:33 AM   #2
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When a photon encounters an atom, there is always the possibility it might or might not interact.
Note that even if the atom has electrons in favourable energy bands, the photon might still fail to interact.

So if the material is only one atom thick many (most) of the photons will get through.
The more layers of atoms the photons are presented with, the more likely they will interact and the less likely they will get through.

The exact thickness required depends on the material being considered and the energy of the photons.
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Old Jan 26th 2016, 09:55 AM   #3
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Seems to me that for metals the material could be very thin - on the order of one wavelength of light, for it to be opaque. Metals have free electrons which I think can interact with photons without too much concern about energy levels, as long as the density of the electron cloud in the metal is similar to the wavelength of light. Of course I might be wrong, and maybe somebody will come up with Mr. Scott's transparent aluminum from Star Trek!

Another material that would be opaque is plasma. The free electrons should interact with photons and would appear opaque.
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Old Jan 28th 2016, 03:47 PM   #4
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Smile Thanks

Thank you for answer
.
Originally Posted by MBW View Post
When a photon encounters an atom, there is always the possibility it might or might not interact.
Note that even if the atom has electrons in favourable energy bands, the photon might still fail to interact.

So if the material is only one atom thick many (most) of the photons will get through.
The more layers of atoms the photons are presented with, the more likely they will interact and the less likely they will get through.

The exact thickness required depends on the material being considered and the energy of the photons.
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Old Jan 28th 2016, 08:30 PM   #5
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Chip is quite correct. The opacity of a sheet of matter is not determined by its thickness but by the material that its made of. A thin sheet of aluminum is opaque to light whereas a sheet of glass 10 feet thing will be transparent.
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