Atom struture : opaque and Transparent

[JPC]

hey

what in an atoms struture tells us if the element is going to be transparent or opaque ?

what in an atoms structure tells us at what frequency photons will come out with after hiting the electrons ?

 

[ZapperZ]

Please read the entry in our FAQ in the General Physics forum.

Zz.

 

[Hootenanny]

A solid has a network of ions and electrons fixed in a "lattice". Think of this as a network of balls connected to each other by springs. Because of this, they have what is known as "collective vibrational modes", often called phonons. These are quanta of lattice vibrations, similar to photons being the quanta of EM radiation. It is these vibrational modes that can absorb a photon. So when a photon encounters a solid, and it can interact with an available phonon mode (i.e. something similar to a resonance condition), this photon can be absorbed by the solid and then converted to heat (it is the energy of these vibrations or phonons that we commonly refer to as heat). The solid is then opaque to this particular photon (i.e. at that frequency). Now, unlike the atomic orbitals, the phonon spectrum can be broad and continuous over a large frequency range. That is why all materials have a "bandwidth" of transmission or absorption. The width here depends on how wide the phonon spectrum is.

On the other hand, if a photon has an energy beyond the phonon spectrum, then while it can still cause a disturbance of the lattice ions, the solid cannot sustain this vibration, because the phonon mode isn't available. This is similar to trying to oscillate something at a different frequency than the resonance frequency. So the lattice does not absorb this photon and it is re-emitted but with a very slight delay. This, naively, is the origin of the apparent slowdown of the light speed in the material. The emitted photon may encounter other lattice ions as it makes its way through the material and this accumulate the delay.

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Edit: Dammit Zz :tongue:

 

[ZapperZ]

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Edit: Dammit Zz :tongue:

Well, you also missed the most important message in that FAQ. :)

The moral of the story there is that "individual atoms" no longer play a significant role in almost all of the obvious properties of a solid. That is something that needs to be highlighted. People continue to ask about atomic absorption when light passes through a solid, and that is just not correct. The optical behavior, at least in the visible range, is now governed by the collective behavior of the solid, not by the atomic level of the individual atoms.

If a solid behave the same way as individual atoms, there would be no need to have a field of study separate and different from atomic/molecular physics. Yet, we do Solid State/Condensed Matter physics.

Zz.

 

[Hootenanny]

Well, you also missed the most important message in that FAQ. :)

Moral of the story: the properties of a solid that we are familiar with have more to do with the "collective" behavior of a large number of atoms interacting with each other. In most cases, these do not reflect the properties of the individual, isolated atoms.

 

[JPC]

yeah i know its not the atom that makes the color, its the molecule
but i was just asking for the case of atoms to simplify things

Btw, thanks for your explanations

well yeah, i could have searched for a topic that would answer my questions , but there are so many of them, and I've only been here recently so

 

[Gokul43201]

yeah i know its not the atom that makes the color, its the molecule

No, it's not the molecule either. In fact, in most solids, there's no such thing as a molecule. Please read the FAQ post more carefully - here's the link: https://www.physicsforums.com/showpost.php?p=899393&postcount=4