Refraction of light -- frequency remaining constant

[logearav]

Revered Members,

When light travels from rarer to denser medium and vice versa, velocity and wavelength changes but frequency remains constant. What is the reason? This question was asked in the physics quiz competition held in my school. I replied that since energy of the light remains the same and since energy is proportional to frequency, frequency remains constant. But the convener said the answer is wrong. Am i wrong?

[swansont]

The constant frequency can be shown by applying the proper boundary conditions to Maxwell's equations, which describe EM radiation. There are continuity requirements for certain field components.

 

The energy of the photon doesn't stay the same, because some of the energy is actually shared with the medium; when an optical fiber absorbs or emits a photon, it recoils, which takes energy and momentum. (It's part of the solution to the Abraham-Minkowski controversy) http://physics.aps.org/story/v22/st20

[Secular Sanity]

What about when the photons do not interact with the electrons by emission or absorption, and pass through the glass unchanged, i.e. transparency? If they are not interacting with the electrons then the photons have not slowed down, right? So what is it exactly that causes refraction ?

 

 

Edited November 20, 2011 by Secular Sanity

[Klaynos]

What about when the photons do not interact with the electrons by emission or absorption, and pass through the glass unchanged, i.e. transparency? If they are not interacting with the electrons then the photons have not slowed down, right? So what is it exactly that causes refraction ?

 

There is still absorption and re-emission, if there was not then the refractive index of the material would 1 and this question doesn't makes sense because all three of the mediums are vacuum. These absorption and re-emission events are what causes the slowing down of the waves, photons always travel at c, they just spend some time absorbed.

[swansont]

I will add that the absorption/emissions are not in real states, so the atom does not end up exchanging any momentum or energy with the photon.

[Secular Sanity]

There is still absorption and re-emission, if there was not then the refractive index of the material would 1 and this question doesn't makes sense because all three of the mediums are vacuum. These absorption and re-emission events are what causes the slowing down of the waves, photons always travel at c, they just spend some time absorbed.

 

Wrong.

 

 

Please don't do this. Linking like this is an off-topic distraction. Give people a chance to get to the post on their own.

Do not respond to this modnote

 

Pfft…how rude!

 

I will add that the absorption/emissions are not in real states, so the atom does not end up exchanging any momentum or energy with the photon.

 

It's taught in intro to electrodynamics. You added to it but can you answer it, swansont?

Edited November 20, 2011 by Secular Sanity

[Klaynos]

 

Wrong.

 

 

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Pfft…how rude!

 

[/size]

 

It's taught in intro to electrodynamics. You added to it but can you answer it, swansont?

 

That video does not cover absorption and re-emission (as swansont added into not real states) that results in the slowing down of the wavefronts.

 

It discuses interband absorptions.

[Secular Sanity]

Hmm…are you implying that you knew the answer all along, Klaynos?

 

 

Not in real states? Pfft...that's an answer? Whatever.

 

The original OP was "When light travels from rarer to denser medium and vice versa,velocity and wavelength changes but frequency remains constant. What is the reason?"

 

 

"As the wave passes through, the fields busily polarize and magnetize all the molecules, and the resulting (oscillating) dipoles create their own electric and magnetic fields. These combine with the fields in such a way, as to create a single wave with the same frequency but different speed. This extraordinary conspiracy is responsible for the phenomenon of transparency and refraction."

 

David Griffiths

 

 

 

 

Edited November 20, 2011 by Secular Sanity

[swansont]

"As the wave passes through, the fields busily polarize and magnetize all the molecules, and the resulting (oscillating) dipoles create their own electric and magnetic fields. These combine with the fields in such a way, as to create a single wave with the same frequency but different speed. This extraordinary conspiracy is responsible for the phenomenon of transparency and refraction."

 

David Griffiths

 

David Griffiths has written some great textbooks and knows what he's talking about. The thing is, that quote in no way contradicts anything Klaynos or I said in the thread. Griffiths gives a description of the classical effect, which is a qualitative restatement of "solve Maxwell's equations with the proper boundary conditions". Klaynos gave a QM description, to which I added.

[Phi for All]

 

Pfft…how rude!

 

[/size]

!

Moderator Note

You were warned not to hijack threads by diverting links to other threads. Similarly, responding to other threads within this one is also distracting. PLEASE STOP NOW.

 

Further, your combative attitude is completely uncalled for. No one has done anything to deserve this kind of treatment. If it continues, you will be suspended immediately.

 

Contradicting your ideas is in no way a personal attack. No one is being rude by pointing out your errors. Please follow the rules you agreed to when you joined this forum!

 

If you further derail this or any other threads by responding to this modnote, you will be suspended immediately.

[Secular Sanity]

David Griffiths has written some great textbooks and knows what he's talking about. The thing is, that quote in no way contradicts anything Klaynos or I said in the thread. Griffiths gives a description of the classical effect, which is a qualitative restatement of "solve Maxwell's equations with the proper boundary conditions". Klaynos gave a QM description, to which I added.

 

Well, his QM description isn't correct.

 

The energy of the photon doesn't stay the same' date=' because some of the energy is actually shared with the medium; when an optical fiber [b']absorbs or emits a photon[/b], it recoils, which takes energy and momentum.

 

There is still absorption and re-emission' date=' if there was not then the refractive index of the material would 1 and this question doesn't makes sense because all three of the mediums are vacuum. [b']These absorption and re-emission events are what causes the slowing down of the waves, photons always travel at c, they just spend some time absorbed.[/b]

 

Sorry, but he clearly states that the answer is due to admission and absorption, which is incorrect. How can you argue with that?

 

So, if you simply say that the absorption/emissions are not in "real states" this makes the answer correct? What? Please explain.

[mooeypoo]

Well, his QM description isn't correct.

You say this because after years of study of QM, you found what evidence to counter his, exactly?

 

Sorry, I'm usually not insisting on a person's level of education when claims are brought up, but in this case, it seems you are way ahead of yourself. Are you here to discuss and learn, or to shove your ideas into agreement even if they don't work?

 

Are you ready to back up your extraordinarily extravagant claims that you know better Quantum Mechanics than Griffits and other PhD physicists (that work and handle QM in daily operation) ?

 

I am not saying this can't be true. I'm saying the burden of proof is on you to show it is. As it is at the moment, you're just arguing (with attitude). You supplied not a shred of evidence to show you have any sort of merit to your claims. All you did is cry foul over established theory with evidence.

 

You're the one making a claim here, and hte burden of proof is on you.

 

Let's work with science and not bombastic arguments, okay? I'm waiting for your evidence.

 

~mooey

[Secular Sanity]

You say this because after years of study of QM, you found what evidence to counter his, exactly?

 

Sorry, I'm usually not insisting on a person's level of education when claims are brought up, but in this case, it seems you are way ahead of yourself. Are you here to discuss and learn, or to shove your ideas into agreement even if they don't work?

 

Are you ready to back up your extraordinarily extravagant claims that you know better Quantum Mechanics than Griffits and other PhD physicists (that work and handle QM in daily operation) ?

 

I am not saying this can't be true. I'm saying the burden of proof is on you to show it is. As it is at the moment, you're just arguing (with attitude). You supplied not a shred of evidence to show you have any sort of merit to your claims. All you did is cry foul over established theory with evidence.

 

You're the one making a claim here, and hte burden of proof is on you.

 

Let's work with science and not bombastic arguments, okay? I'm waiting for your evidence.

 

~mooey

Why on earth would you say this? No, dear Mooeypoo, I'm not claiming to know QM better than David Griffiths. Hello! I posted an answer from Griffiths book "Introduction to Electrodynamics".

 

Everything else you said was just um...um...hmm...well, let's just leave it at that, shall we?

[Klaynos]

I refer you to:

 

QED: The Strange Theory Of Light And Matter by R. Feynman.

 

There are many difference in descriptions between classical electrodynamics (what you quoted) and quantum electrodynamics (what I was describing).

[swansont]

Why on earth would you say this? No, dear Mooeypoo, I'm not claiming to know QM better than David Griffiths. Hello! I posted an answer from Griffiths book "Introduction to Electrodynamics".

 

Everything else you said was just um...um...hmm...well, let's just leave it at that, shall we?

 

Which is a very fine undergraduate text in the area of classical physics.

[Secular Sanity]

I refer you to:

 

QED: The Strange Theory Of Light And Matter by R. Feynman.

 

There are many difference in descriptions between classical electrodynamics (what you quoted) and quantum electrodynamics (what I was describing).

 

Okay. Now, it would be nice if you threw in something from that book that backs up what you're saying.

[Klaynos]

Okay. Now, it would be nice if you threw in something from that book that backs up what you're saying.

 

Well to understand it fully I'd suggest you read the whole book. But chapter 3 explains it well.

 

Quoting selected text will not help anyone without the background and understanding presented in the rest of the book.

[Secular Sanity]

Well to understand it fully I'd suggest you read the whole book. But chapter 3 explains it well.

 

Quoting selected text will not help anyone without the background and understanding presented in the rest of the book.

 

These absorption and re-emission events are what causes the slowing down of the waves, photons always travel at c, they just spend some time absorbed.

 

Oh, I have that book, silly me. Ah yes, Chapter 3, here it is.

 

Recipes for Word Salad by Klaynos

 

Blah, blah, blah...

 

 

Thanks for clearly that up.

 

Cheers!

What causes the slowing down of the waves?

The explanation that Klaynos provided is basically that a photon moving through the material still moves at the speed of c, but when it encounters the atom of the material, it is absorbed by the atom via an atomic transition. After a very slight delay, a photon is then re-emitted. This explanation is incorrect and inconsistent with empirical observations.

Edited November 21, 2011 by Secular Sanity

[mooeypoo]

Hm.

 

Okay, here's the bottom line here: I misread your earlier comment. While I do apologize, that does not give you the right to ignore our requirement in this forum that you drop your attitude, stop being obnoxious, and start debating in a civil matter. You can be the smartest person on earth, and we would still ban you if your posts are full of mockery.

 

We don't care who's organs are supposedly larger, Secular Sanity, we're here to do science, and our "requests" that you be civil are not really requests, they're our rules. Rules you agreed to when you checked that little box when you signed up. They're non-negotiable.

 

If you insist on disregarding them, you won't continue to be a member here. You will be banned, regardless of whether or not you're right or wrong.

 

Ridicule is unacceptable whether you argue with someone who made a mistake or argue with someone who actually cares to put the time and supply counter-arguments to your claims.

 

Cheers.

 

Consider this your final warning.

[Klaynos]

 

The explanation that Klaynos provided is basically that a photon moving through the material still moves at the speed of c, but when it encounters the atom of the material, it is absorbed by the atom via an atomic transition. After a very slight delay, a photon is then re-emitted. This explanation is incorrect and inconsistent with empirical observations.

 

I would rather you didn't misquote me, I never said atomic transitions. Photons always move at the speed of light. Please re-read what I wrote, and I would implore you to seek out the book I suggested. No matter what level of education you have it is a fantastic and concise text on quantum electrodynamics. I'd imagine your local library will have a copy of it. It is the text from a short series of lectures that Feynman presented, I cannot hope to cover the full content on an internet forum.

[Secular Sanity]

I didn't misquote you, Klaynos. Here's what you stated...word for word.

 

These absorption and re-emission events are what causes the slowing down of the waves, photons always travel at c, they just spend some time absorbed.

 

Please note that all posts that are baseless in scientific fact or that are outside of mainstream physics can and will be moved to the Speculations forum.

 

Perhaps, they can simply place your statement in the Speculations forum. Then mooeyspoo will have more room to do his science.

 

 

 

Edited November 21, 2011 by Secular Sanity

[Klaynos]

I didn't misquote you. Here's what you stated...word for word.

 

 

Perhaps, they can simply place your statement in the Speculations forum. Then mooeyspoo will have more room to do his science.

 

 

 

And where are the atomic transitions mentioned? This is a little off topic.

 

You are yet to present any corroboration that my statement (with the clarification of swansont's comment) is wrong. The quote of Griffiths is classical electrodynamics but even so seems to support my quantum electrodynamical explanation to which I have provided a reference. Your video as I stated was talking about interband transitions not the non-real very short lifetime transitions for which the quantum explanation relies.

[Secular Sanity]

Not the non-real very short lifetime transitions for which the quantum explanation relies.

What?

 

Let's KISS then, shall we? This is what you said "These absorption and re-emission events are what causes the slowing down of the waves, photons always travel at c, they just spend some time absorbed." Is this your explanation of why a photon slows down in a medium?

 

Edit: I was still able to log on. I haven't been banned, yet...bwahahah!

If your statement were correct, the index of refraction would only depend on the type of atoms in the material. Graphite and diamond are both made of carbon atoms, and yet their index of refraction is different. Are you attempting to imply that you were referring to the de Broglie or Schrödinger's wave?

"Kiss is often wrong." That's because you've never been kissed by me. Mmmwha!...kiss,kiss.

See ya!

 

Edited November 21, 2011 by Secular Sanity

[Klaynos]

Kiss is often wrong. If you really care I still suggest you read the book I refer to.

 

No, atomic type isn't the only important thing, and nothing I've said has stated that. It is quite complicated and there are lots of different permittivity models for different material types, if anyone is interested I'd suggest another thread. These different models use a range of.different qm and classical approximations.

 

No, atomic type isn't the only important thing, and nothing I've said has stated that. It is quite complicated and there are lots of different permittivity models for different material types, if anyone is interested I'd suggest another thread. These different models use a range of.different qm and classical approximations.

[Mellinia]

Please give use the other thread. We could use with some on topic reading.