Reflection And Refraction

[Lancewen]

How is reflection of a photon different or similar to refraction at the atomic level? What actually happens that causes the photons to be readmitted at the angle it does?

 

I know when a photon is reflected it is always at the same angle as the incoming photon. But a refracted photon is readmitted at a different angle that depends on the composition of the transparent material. I believe the term is called refraction index. Why does this happen at the atomic level?

 

 

[Qfwfq]

How is reflection of a photon different or similar to refraction at the atomic level?

Similar principles of wave optics apply, one needs to know how well you already understand them.

 

But a refracted photon is readmitted at a different angle that depends on the composition of the transparent material. I believe the term is called refraction index.

The way atoms and molecules interact with the oscillating field has the overall effect of slowing the propagation of the wave. Phase relation computations give the rest.

[Lancewen]

Similar principles of wave optics apply, one needs to know how well you already understand them.

 

Was looking for a non math answer if one is possible.

 

The way atoms and molecules interact with the oscillating field has the overall effect of slowing the propagation of the wave. Phase relation computations give the rest.

 

Here I notice you are talking about a wave as opposed to a photon. Is it possible to explain the slowing propagation in terms of what happens to a single photon?

[Qfwfq]

Was looking for a non math answer if one is possible.

With some geometry at the very least. Look up stuff about optical path length.

 

Here I notice you are talking about a wave as opposed to a photon. Is it possible to explain the slowing propagation in terms of what happens to a single photon?

Only in terms of probability.

[Lancewen]

Only in terms of probability.

 

Probability? Not sure I understand that answer when refraction index is always the same for any given transparent material. The speed of propagation and angle of refraction is very precise. Where does probability come into any answer of what happens between atoms and photons that always produce the same result every time?

[JMJones0424]

Probability? Not sure I understand that answer when refraction index is always the same for any given transparent material. The speed of propagation and angle of refraction is very precise. Where does probability come into any answer of what happens between atoms and photons that always produce the same result every time?

 

This is a common misconception. I wish I understood it well enough to succinctly explain, but I don't. I can, however, recommend a book that explains it far better than I could hope to do, The Strange Theory of Light and Matter by Richard Feynman. It's available free online at Scribd:

 

http://www.scribd.com/doc/30975492/The-Strange-Theory-of-Light-and-Matter-by-Richard-Feynman

 

and likely at a local library or at Amazon. Simply put, what you think of as an exact reflection (or refraction) of a photon is actually a summation of probabilities.

 

If you'd prefer to hear it straight from the man himself, you can watch the four lectures (about an hour and a half each) that the book is based on here:

http://www.vega.org.uk/video/subseries/8

Edited March 15, 2012 by JMJones0424

[Lancewen]

This is a common misconception. I wish I understood it well enough to succinctly explain, but I don't. I can, however, recommend a book that explains it far better than I could hope to do, The Strange Theory of Light and Matter by Richard Feynman. It's available free online at Scribd:

 

http://www.scribd.co...Richard-Feynman

 

and likely at a local library or at Amazon. Simply put, what you think of as an exact reflection (or refraction) of a photon is actually a summation of probabilities.

 

If you'd prefer to hear it straight from the man himself, you can watch the four lectures (about an hour and a half each) that the book is based on here:

http://www.vega.org....deo/subseries/8

 

Thanks, I will. I knew there was a good chance my question was going to be hard to answer, but i really like how the scientists on this forum treat people who ask dumb or very difficult questions as opposed to other forums I've been on. I consider you posting this information as going the extra mile and it is appreciated.

[Qfwfq]

arKane, I think your best starting point is to look up the optical path length, a google ought to give you results that you could choose according to what you best understand, because it is really a simple enough concept, the length depends on the speed as well as the geometric distance and so it is proportional to the elapsed time. At that point you could go on to Fermat's principle and how it relates to reflection and reraction, perhaps in this page:

 

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/fermat.html

 

Probability? Not sure I understand that answer when refraction index is always the same for any given transparent material. The speed of propagation and angle of refraction is very precise. Where does probability come into any answer of what happens between atoms and photons that always produce the same result every time?

It's rather complicated and I would say it should come a bit later if you find these things dificult. In essence, the wave computations are precise but in general they don't tell each corpuscle exactly what to do, they serve to compute probabilities which ive a statistical distribution for the whole herd.

[CraigD]

How is reflection of a photon different or similar to refraction at the atomic level? What actually happens that causes the photons to be readmitted at the angle it does?

According quantum electrodynamics (QED), the short answer is that nothing causes a photon to be reemitted at a specific angle when it’s absorbed then reemitted by an electron in a reflective or refractive material. The photon has an equal probability of being emitted in any direction, by an electron in any position, and it is this probability, not our classical imagining the unobserved photon being emitted at a definite single angle, that is real.

 

When one calculates, following correct statistical rules, the sum and products of all the probability of a photon being emitted in all possible directions by all possible electrons (as there are an infinitude of these, this is technically complicated – enough to win a Nobel prize), one finds that the probability of it being detected at precisely the point and time expected by it being emitted at precisely the angle given by the laws or reflection and refraction and traveling at the speed of light is nearly 100%, while the probability of it being detected somewhere/when else is nearly 0%.

 

As did JMJones in this post, I enthusiastically recommend you read Richard Feynman superb 1983 QED: The Strange Theory of Light and Matter. You may view and hear the actual lectures of which this book is simply a transcript at this site. Either way you do it, I recommend you expect to spend several days at it – as Feynman notes, even without learning the techniques that take a physics grad student years to learn, a decent grasp of QED takes at least a few days to acquire.

 

I know when a photon is reflected it is always at the same angle as the incoming photon. But a refracted photon is readmitted at a different angle that depends on the composition of the transparent material.

If you follow what I’ve summarized above – or better, read Feynman or another good popularize (beware, though, as not all of them are) – you should understand that this isn’t, really, what happens on the very small scale of photons, electrons, and nuclei (which QED doesn’t explain in detail, handling only as approximately stationary positively charged bodies). The detection of the photon indicates that it follows the laws of reflection and refraction, but this measurement is determined by the interaction of the probability associated with the detected photon and the electrons of the reflecting or refracting body (mirror, lens, etc.).

 

Why does this happen at the atomic level?

I can still remember my frustration at being told “reflection and refraction is all explained by QED – go learn about the math, and you’ll understand.” (As this started happening to me in the 1970s, before a hyperlink could give me instant access to material like Feynman’s lectures, and when few educated adult could give even as much guidance as “learn QED,” I think I had it worse then than people who haven’t yet gained a good layperson’s grasp of QED have it now!) So, in sympathy, I’ll offer this directly, semi-classical terms (treating photons and electrons like way everyday objects like apples and bullets, with QED as a kind of “ghost effect”) explanation of reflection, with warnings not to take it as more than a sort of comforting allegory.

 

A single photon encounters an electron in a reflective material, and is absorbed by it. The electron emits a photon of the same energy (and thus the same wavelength) as the one absorbed. The “ghost” of how the photon could have been absorbed and emitted by all the other electrons in the reflector interferes with the emitted photon to require it to be reflected at the same exit angle as its entry angle.

 

This semi-classical “ghost” effect, which is just an obscured packaging of the probability mechanics of QED, is key to how reflectors work. Consider that, semi-classically, the photon interacted with a single electron in a single atom (or perhaps a few, if it missed those in the outermost layer). This atom doesn’t “know” anything about the orientation of the surface of the reflector of which it’s a part, so it’s electron doesn’t “know” how to emit the photon at the correct angle. The “ghost” effect “knows”.

[belovelife]

hmmm,

 

cool stuff i wrote a poem about this once