How does photon react in reflection and refraction?


by null void
Tags: photon, react, reflection, refraction
null void
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#1
Dec7-13, 06:57 AM
P: 79
I want to apologize first of i post at the wrong section, not very sure if this is the right place for this post.

First question, does the photon get absorbed when it hit highly reflecting material like mirror ? What make them look so different from the other transparent object? And I just read about the black body radiation, not sure if i get the right idea or not, but this is kind of confusing...Inside the thermos, it has a layer of shiny material, and they reflect radiation right? But when we close the opening, there is no light in the thermos, can i consider it as a black body? Or the radiation from the hot water is consider as a kind of "light" ?

In refraction, i suppose that the photon pass through the object, the particle of the object didn't absorb the photon? Or they did but they emit them in the same direction? If so why don't reflecting material does the same thing?
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Claude Bile
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#2
Dec7-13, 10:12 AM
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Photons are not absorbed in the case of either reflection or refraction.

Claude.
phyfun
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#3
Dec8-13, 10:24 PM
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Black body is a particular thing that absorb Electromagnetic waves without any reflection, and it also radiate Electromagnetic waves. Close the opening of thermos does not mean it become a black body. On the contraty, the opening can be seen as a black body.

ChrisVer
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#4
Dec9-13, 01:50 PM
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How does photon react in reflection and refraction?


I think, in a classical view point, you can always consider the photons to be absorbed and re-emitted in each point in spacetime. In fact that would lead you in Huyghen's principle...
In the classical langrangian viewpoint, you'd get that the photons are propagating in their classical trajectories, which are the ones you see, and they are the ones minimizing your action.
kurros
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#5
Dec9-13, 04:26 PM
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Quote Quote by null void View Post
First question, does the photon get absorbed when it hit highly reflecting material like mirror ?
Well, what 'really' happens is extremely complicated, but yeah I think it is not a lie to say that the incident photons are absorbed and reemitted. But technically you can say this when a photon interacts with anything.

Quote Quote by null void View Post
What make them look so different from the other transparent object?
Mirrors aren't transparent, so I don't really understand the question. They look different to other opaque materials for several reasons: 1. Their microscopic surface is really smooth (so the reflected light doesn't get scattered off in all directions like with most surfaces); 2. They are really "white", i.e. they reflect most visible wavelengths of light pretty equally; 3. I think the fact that they are made from metals and have mobile electrons floating around their surface is relevant too, but I don't quite remember why... this might be why they can reflect all different wavelengths so well.

Quote Quote by null void View Post
In refraction, i suppose that the photon pass through the object, the particle of the object didn't absorb the photon? Or they did but they emit them in the same direction? If so why don't reflecting material does the same thing?
I don't think I can answer this, it is too hard for me. Might need a proper optics person.
Cthugha
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#6
Dec10-13, 03:36 AM
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Quote Quote by null void View Post
First question, does the photon get absorbed when it hit highly reflecting material like mirror ? What make them look so different from the other transparent object?
No. It is a common myth that photons are absorbed and reemitted when passing through a material or getting refracted or reflected, but that idea is wrong and inconsistent with experiments. Some variants of this question arise so often that we have a FAQ entry on the question whether photons are constantly absorbed and reemitted in a medium:
http://www.physicsforums.com/showthread.php?t=511177

Reflection at a mirror roughly works as follows: Mirrors are usually made from metals which have free electrons. Light fields create a force on electrons, so they are displaced from their equilibrium position. This in turn creates an inhomogeneous distribution of electrons which creates a Coulomb force which acts in order to restore the equilibrium distribution. Now there are two possible situations. Either this electron plasma as a whole can oscillate as fast the the light field does or it is slower than that. If it can oscillate as fast as the light field does, the oscillating charges will in turn create a second light field (accelerated charges radiate) which is superposed with the incident light field and exactly cancels the incident light field in the forward direction. However, the component in the backward direction still persists and gives the reflected beam. If the electron plasma is not able to oscillate that quickly, the metal is more or less transparent for the light. For most metals, this plasma frequency roughly corresponds to light fields in the UV, somewhere around 300 nm, so UV light passes through, while visible light gets reflected. The slightly different plasma frequencies also play a role in the different colors different metals have.

It is important to note that all of this is a coherent process. The phase is preserved, while it would be randomized if absorption and emission took place.
siddharth5129
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#7
Jan5-14, 01:39 AM
P: 58
Quote Quote by Cthugha View Post
No. It is a common myth that photons are absorbed and reemitted when passing through a material or getting refracted or reflected, but that idea is wrong and inconsistent with experiments. Some variants of this question arise so often that we have a FAQ entry on the question whether photons are constantly absorbed and reemitted in a medium:
http://www.physicsforums.com/showthread.php?t=511177

Reflection at a mirror roughly works as follows: Mirrors are usually made from metals which have free electrons. Light fields create a force on electrons, so they are displaced from their equilibrium position. This in turn creates an inhomogeneous distribution of electrons which creates a Coulomb force which acts in order to restore the equilibrium distribution. Now there are two possible situations. Either this electron plasma as a whole can oscillate as fast the the light field does or it is slower than that. If it can oscillate as fast as the light field does, the oscillating charges will in turn create a second light field (accelerated charges radiate) which is superposed with the incident light field and exactly cancels the incident light field in the forward direction. However, the component in the backward direction still persists and gives the reflected beam. If the electron plasma is not able to oscillate that quickly, the metal is more or less transparent for the light. For most metals, this plasma frequency roughly corresponds to light fields in the UV, somewhere around 300 nm, so UV light passes through, while visible light gets reflected. The slightly different plasma frequencies also play a role in the different colors different metals have.

It is important to note that all of this is a coherent process. The phase is preserved, while it would be randomized if absorption and emission took place.

Thanks a ton. That really cleared it up for me. I've been looking everywhere for an answer to this question. Just a few clarifications. If the plasma frequency of most metals corresponds to UV, shouldn't metals reflect UV ? You mentioned that if the electron plasma can oscillate as fast as the light field, then the component in the backward direction persists and gives reflection.

Secondly, if the electron plasma cannot oscillate as quickly as the light field, which gives more transmission, a smaller oscillation frequency, or a larger one ? And how does light get transmitted for a completely transparetn material ? Is it that it just doesn't interact with the plasma ? And further, what electronic properties of materials decide whether they are transparetn or opaque ?
Cthugha
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#8
Jan6-14, 04:59 AM
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Quote Quote by siddharth5129 View Post
If the plasma frequency of most metals corresponds to UV, shouldn't metals reflect UV ? You mentioned that if the electron plasma can oscillate as fast as the light field, then the component in the backward direction persists and gives reflection.
The plasma frequency is the frequency where propagating modes just start to appear in the medium and the electrons stop being able to follow the external driving frequency. If the frequency is lower (higher wavelength), you get reflection.

Quote Quote by siddharth5129 View Post
Secondly, if the electron plasma cannot oscillate as quickly as the light field, which gives more transmission, a smaller oscillation frequency, or a larger one ? And how does light get transmitted for a completely transparetn material ? Is it that it just doesn't interact with the plasma ? And further, what electronic properties of materials decide whether they are transparetn or opaque ?
These are a lot of questions. Transparent materials (in the visible regime) are rather dielectrics than metals and thus do not have a plasma, but relatively bound carriers.

Also, see the FAQ for some really good explanations and post again if you have detailed questions:
http://www.physicsforums.com/showthread.php?t=511177


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