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Author Topic: What properties makes something transparent?  (Read 2125 times)

Offline thedoc

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What properties makes something transparent?
« on: 02/08/2013 02:30:03 »
Roderick Chia asked the Naked Scientists:
   
The concept of transparency has been a great mystery to me. I originally suspected that a substance is transparent if the atoms in the substance are far a part and opaque if the too close together. However, knowing that diamond a higher density than graphite, this hypothesis doesn't work. It is also clear that water appear more transparent than stream, which is technically less dense. Perhaps transparency is a relative terms depending of the colour of light.

Can you shed some light on this?
What do you think?
« Last Edit: 02/08/2013 02:30:03 by _system »


 

Offline yor_on

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Re: What properties makes something transparent?
« Reply #1 on: 02/08/2013 13:08:54 »
That one is tricky. I would also want a clear answer to that one, but it's theories, some treating light as waves, other as photons, and yet others as 'fields' with 'ripples' that then express itself as particles, as photons.

One definition is that you have discrete energy levels resting in each atom, depending on the 'orbitals' of its electrons. They can then either become traps absorbing the photons, depending on materials energy levels and the photons frequency, or when the frequency is wrong for absorbing let them pass through, Whereas a atom only contain some few frequencies a molecule contain a lot of atoms, and so may have a lot of different frequencies able to absorb different (energy states) photons. Reasoning like this we then find that photons can pass through glass when their frequency is wrong for being absorbed, as the electron want a well defined energy, enabling it to 'jump' from one energy state to another ('discrete jumps' instead of a continuous scale, look up blackbody radiation for that one). And depending on what light you use and its energy, all materials can become transparent as I get it. Just think of a x-ray showing bones. but no flesh. And when it's not transparent it either reflects photons back or absorb them, transforming the excess into heat, or do both, depending on material and light (radiation).

Although it is wrong :) at least it isn't the whole truth. Because if you measure the speed of light passing glass you will find it to be slower than it is in a vacuum. If it is slower then it has to interact with the glass. So how can it keep the information being absorbed, to then be re-emitted?

Because what you see looking through a (perfectly transparent and undistorting) glass pane should ideally be a exact replica of the information you would get, opening that same window to look. There are several theories considering how atoms (electrons etc) treat that, and deciding which one that is more correct?
   

"A laser beam passing though transparent glass does not lose energy (or momentum) consequently there's no reason for it to become incoherent. But this is not directly related to the question of whether there is scattering or absorption and delayed emission.

There's a difficulty in relating what happens on a photon by photon basis with what happens in the classical limit. You know that light traveling through glass is slowed down, and it seems that this must be due to a delay between absorption and emission, and I suppose that this seems to imply that there must be a delay between absorption and emission in the quantum analysis of the situation.

"Quantum mechanics" is limited to situations where the particles are neither created nor destroyed; this can't include absorption and emission. And it's not terribly easy with large number of particles in the same state (as happens in a laser). The more general "Quantum Field Theory" allows creation and annihilation of particles (photons) so this is what's appropriate.

The quantum field theory that applies to photons interacting with electrons (atoms) is quantum electrodynamics (QED). In this theory, the speed of photons is random; they do not have to travel at speed c. So it's possible for a QED problem to have a delayed photon without a delay between absorption and emission (but the average speed will still be c, as far as I know).

A crystal (or other solid) lattice contains a large number of atoms. What determines whether or not a photon can be absorbed is whether the crystal lattice has an available vibration corresponding to the energy, momentum, and angular momentum of the incoming photon. If it does, then the solid will absorb that photon and it will not be transparent in that frequency.

If a photon has an energy that is not compatible with the crystal lattice then it must adjust its energy by borrowing energy. This can only happen for a time ΔEΔt≈ℏ. One supposes that energies where ΔE is small should have a slower speed of light (larger index of refraction)." By Carl Brannen

You can apparently also think of photons as becoming entangled with 'phonons' (quantums of vibrational energy, having a direction and speed) in a thought up lattice.

"You don't need to lose energy or momentum to become incoherent, you only need to get entangled with something. A photon which knocks a phonon into a different state will become incoherent with other photons (this process is suppressed if there are other photons around), because it is entangled with the phonon direction. The absorption/reemission off resonance is identical with scattering, since you can describe scattering as a sum over the intermediate available states of absorption followed by reemission, in an appropriate framework. Ron Maimon"

Let's see what JP has to say :) It's one of the mysteries to me.
« Last Edit: 02/08/2013 13:20:13 by yor_on »
 

Offline yor_on

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Re: What properties makes something transparent?
« Reply #2 on: 06/08/2013 15:48:46 »
The really interesting question, to me in this, is. What is that 'information' able to pass through a material as glass? And is it a replica of the same information without a glass?

How do information transfer here? Can one break it down in some simple parameters covering what it is I see? Somehow everything can be seen as transparent it seems, depending of 'energy' applied. So, what is it I see?
 

Offline syhprum

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Re: What properties makes something transparent?
« Reply #3 on: 06/08/2013 19:51:38 »
It always surprises me that you can make transparent glass from lead seemingly the densest of materials.
 

Offline evan_au

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Re: What properties makes something transparent?
« Reply #4 on: 06/08/2013 22:19:52 »
Quote
water appears more transparent than steam
  • Water has a uniform density, so light will pass through it in a straight line, forming a clear image of what is on the other side of a fishtank.
  • Really hot Steam, well above 100C consists of pure water vapour, and is quite transparent.
  • Steam from a kettle (or fog) consists of tiny droplets of water dispersed in air. When light strikes a droplet of water, it is scattered in different directions, with different colours traveling in slightly different directions. This hides the shape of distant objects, without absorbing the light. Steam & Fog appear white.
  • Snow (plus Sugar & Salt) consists of tiny crystals. When light strikes the crystals, it is scattered in different directions, again without absorbing the light. Piles of clear crystals appear white.
Technically, all these different arrangements of water are all transparent (ie the light is not absorbed). But the scattering of the light on droplets and crystals make it translucent (ie you can't see an image clearly).
 

Offline yor_on

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Re: What properties makes something transparent?
« Reply #5 on: 07/08/2013 18:56:36 »
Yes, but the strange thing, and the one I haven't found defined (maybe it is though?) is how information passes through that glass? We make a vista, define it at every point by parameters, put a perfectly transparent undistorted glass in-between, then compare parameters. If they don't differ, am I missing something here? And no, you can't refer it to the speed being different, I think?
 

Offline yor_on

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Re: What properties makes something transparent?
« Reply #6 on: 09/08/2013 18:02:28 »
I think this questions really is..

What makes information?
 

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Re: What properties makes something transparent?
« Reply #6 on: 09/08/2013 18:02:28 »

 

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