Diffraction... why does it work?

[thraxes]

I have been doing work in the physics lab at university with this effekt in the last few days and am beginning to wonder, what is the low level explanation to this effekt? I have the higherlevel explanations all checked out (Huygens Effekt etc...), done the calculations, compared them to my measurements and got the proverbial t-shirt . To keep it simple I will be happy with an explanation for a monochromatic beam.

Put simply, why do the Photons alter their angle of travel... I know the how and with which theories you can calculate this but why do the photons see it necessary to create such an effect and befuddle innocent students like me?

 

[Rainsford]

Not sure if this is what you are asking, but it's because the Photon is not only a particle, but also a wave, at the same time. It doesn't make any sense (at least to our lower level way of thinking), so don't try to understand how that is possible. I think there is some missing link there to be discovered that will make it all make sense. In fact, everything is a wave and a particle, even ourselves. But as for your question, it's becaus the photon is a wave and a particle. Or a particle with wave properties. So, through interference (constructive and destructive) you get the diffraction pattern. It makes perfect sense if it is a wave, but to be honest, I just got done with this stuff in my Physics class, and I'm not sure I understand it fully. Like I said above, I think there is a missing link that explains the why a little better.

 

[kylebisme]

well we think about light as a wave and a partcle beucase that is how we see it, but that is not to say that that is actualy how it is. i don't think there will ever be some magic thing "to be discovered that will make it all make sense" although i imagine we can gain deeper understanding into the nature of light. as for and explanation of why it refacts on a low level; "becasuse god made it that way" is just as good a guess as any.

 

[thraxes]

Good to see I am not the only one stumped at this... I guess it is one of the little things that can't really be explained. I can accept that, but it would have been nice if somebody knew the answer... thanks anyway!

 

[RossGr]

If, as you claim, you have done the math what more explaination could you possibly need? Perhaps all you need to do is stare at the math of the problem until it makes sense. This is simply the sum of intesities on the screen of all wave fronts. I am surprised that you have not done the single and double slit experiments with water in a wave tank. The physical relationships which give rise to difraction are made pretty clear there.

; "becasuse god made it that way" is just as good a guess as any.

LOL, is this, or is this not, the High Tech forum? Such responses as this belong in OT.

 

[silverpig]

Also remember that just because the photon is detected at a certain point doesn't mean that 100% of it is there. It's "fuzzy"

 

[Rainsford]

Math does not explain the why, it simply explains the how. That's why I've never been a big fan of "just learning the math". For example, we have a very easy time explaining how gravity works with math, yet no one has figured out why gravity works. The how leaves an incomplete answer.

And the water tank example, while demonstrating the effects of diffraction, it not a 100% accurate demonstration. Light is unique in that it is not only an EM wave in space (like the pressure wave in the water), but that it also behaves like a particle (which the water waves do not). The question isn't why in the diffraction experiment light acts like a wave, but why it does not act like a particle. Or something, it's getting a little later

 

[Jeff7]

I think there is some missing link there to be discovered that will make it all make sense. In fact, everything is a wave and a particle, even ourselves. But as for your question, it's because the photon is a wave and a particle. Or a particle with wave properties.

I've also wondered about the "light is a particle and wave" thing; it's almost like the particle component is orbiting some invisible companion - that way, it's a particle, but its motion through space makes this orbiting appear as a wave. Just a passing thought.

 

[RossGr]

The interesting thing is, the ability now exists to generate single photons. When the double slit experiment is done by firing single photons you see the same diffraction pattern devleope photon at a time.

The trouble most people have with visualization of photons is that they attempt to view them as analogys of physical objects, balls, points or something that we are familiar with. The fact is any such visualization, while it may work for some phenomena, will not work for ALL observed behavior of photons. This, for the simple reason that photons are not like anything that exists outside the quanutm realm. According to the MATH of QM a photon is a localization of a wave packet, by its very nature a wave packet is nonlocalized, there exists a certian propability that it exists at ANY point in space. It is the wave packet nature that permits a photon to behave as wave and an particle.

I will maintain that the fundamental understanding of the behavior of ANY physical system lies in the math. If you can understand the math then you understand the system. Anything else is arm waving and imparts only partial understanding.

Physics and Math cannot tell us WHY something occurs. It can only allow use to analyis what happens and allow us to pretict behavior accordingly.

 

[rimshaker]

Because of wave-particle duality. Remember? Light has both EM and particle characteristics.

 

[kylebisme]

Originally posted by: RossGr
I will maintain that the fundamental understanding of the behavior of ANY physical system lies in the math. If you can understand the math then you understand the system. Anything else is arm waving and imparts only partial understanding.

we devlop mathmatical expresions to explain systems; that does not make math fundemental to the system, only our understanding of it.

 

[RossGr]

Some claim that god must be a mathematician. How else could our mathematical models work so well. Without the math we have no possibility of applying our knowledge, without application, knowledge is useless there for without math physics is useless.

If you can understand the Math that underlies all of our Physical models you have a true understanding of the system. I am not talking about imperial data fitting type models which provide little or no understanding of the physical system but systems like, E&M, GR, SR and QM which are based on a set of differential equations which have been carefully derived from fundamental physical properties. It is very convient that all we can observe are changes and that differential equations are a mathmatical expression of changes.

 

[kylebisme]

Originally posted by: RossGr
If you can understand the Math that underlies all of our Physical models you have a true understanding of the system.

no, you have an understanding of our mathmatical interpitation of the system, this by no means equates to a "true understanding".

 

[silverpig]

I am pretty sure there is no such thing as "acting like a particle" as electrons also diffract. If you could shoot people through a large enough slit, they would also diffract, even if you shot them one at a time. The effect would be so miniscule that it wouldn't be measurable, but it would happen.

 

[thraxes]

Rainsford and Snowman hit the nail on the head. The math explains how it should work and those equations are verified in experiments... so far so good. But it still does not explain the fundamentals of why something works. I admit I am not a ig fan of theories, I tend to think a little more in the practical area.

Obviously since the why of this problem goes into quantum physics, the practical part of me has to take a back seat since this area is all just theoretical... perhaps that is the reason why I don't understand it.

 

[bwanaaa]

I agree. Those guys that invented quantum mechanics were frustrated by the poor state of technology. They had to invent a sloppy form of math to describe what their instruments measured-stuff based on probability. that's why quantum mechanics is SO unenlightening.

I reconcile the 2slit experiment with the following analogy. Imagineyou are lying on a beach which is perfectly straight. Waves approaching this beach are also perfectly straight. Each one appears to you as a a long rolling cylinder approaching the beach and crashing on the beach in one single event.

Imagine now that someone builds a hugedrainage pipe from your hotel straight out to sea. The wave as it approaches the shore will higher where the pipe is. the wave will appear as a bell curve with 2 long flat tails.

Imagine now that your vision is poor so you cannot see small waves. All you see is the wave coming in over the drainage pipe, but you know it is much longer, spanning in fact the entire beach. When you do interference experiments you can detect the presence of the wave in places where you cannot see it.

So it is with photons. Until we develop mastery over quarks and neutrinos to use as our probes, we will never havean accurate picture of what's going on.

 

[RossGr]

I agree. Those guys that invented quantum mechanics were frustrated by the poor state of technology. They had to invent a sloppy form of math to describe what their instruments measured-stuff based on probability. that's why quantum mechanics is SO unenlightening.

Would be interesting to know what qualifications you have to make such a judgement. Why is it that QM remains the most precise theory in existance?

The fact is modern Physics simply does not addess the question of WHY things happen. We observe and analyze, When properly modeled,as in E&M, the math gives amazing insite.

Now the math and understanding of the double slit and diffraction grating is pretty straight forward and does not require QM to explain. It can all be done with the wave model of light and is simply the superpostion of waves. If you cannot understand it by watching water waves then I am not sure how to help you.



Perhaps you are not asking the right question.

no, you have an understanding of our mathmatical interpitation of the system, this by no means equates to a "true understanding".

Needless to say I do not agree with this, while the understanding may not be perfect, the better your math the better your understanding. Arm waveing simply does not cut it.

 

[heliomphalodon]

I definitely agree with RossGr on this one.

You have to decide what you mean when you say that you want to "understand" a system. Some may be disappointed to learn that physics doesn't lead to the kind of "understanding" that they seek. I would say that these folks are indeed asking the wrong questions. Physics is concerned with predicting the outcomes of experiments. Physics says "what" will happen -- the issue of "why" is properly the province of metaphysics.

Do the equations that govern diffraction (or refraction, for that matter) explain "why" the effect occurs? Of course not -- not in the sense of a metaphysical explanation. However, this is not a shortcoming -- because the equations do in fact correctly predict the results of experiments. That is all that physics will ever do! Even a fully-unified and complete physical theory (if such a thing could exist) would "fail" to provide the metaphysical "why" that some folks desire.

To ask "why" photons diffract, and then to reject the equations (despite their predictive power) as insufficient, is to make a category mistake. It is to ask for an explanation in the form of an analogy -- but such an explanation cannot be accurate. A photon exhibits some of the properties of a macroscopic wave, and some of the properties of a macroscopic particle -- but it is neither. It is a photon. The electron also exhibits the so-called "wave/particle duality" as de Broglie suggested.

The double-slit experiment is especially fascinating when performed with individual photons separated in time. For myself, I interpret the result to mean that each photon must take every possible path from the source to the target. Others have different interpretations -- but the equations that govern the physics give only one answer. Only the interpretations are "free" -- the results are constrained by reality. Reality (the result of the experiment) is correctly predicted by physics. Bingo! That is success. It's all that physics has to offer -- but it's plenty!

 

[bwanaaa]

I was especially impressed by Feyneman's explanation of reflection-that gave me a sense of 'why' the reflected angle is equal to the angle of incidence.