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Author Topic: What is diffraction ?  (Read 1504 times)

Offline McQueen

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What is diffraction ?
« on: 31/07/2015 14:08:59 »
Shouldn’t anything that is sufficiently small, solid or otherwise, and possesses periodicity,  passing through a narrower than wave-length opening result in a diffraction pattern ?



What does this say about the Davisson Germer experiment that was supposed to conclusively prove that wave-particle duality actually existed.  Look at the maths of the De Broglie matter wave:
fa51a54a9435b93b80e3fcfc846d0bc4.gif
 What it means  is that if  something is sufficiently  small (sub-atomic) and moves fast it has a ‘matter’ wave- length that becomes significant whereas if the  object is macroscopic and moving at low velocity  the ‘matter’ wave-length becomes so small (on the order of 7603ebe077f2665cf0d12228cecf26f2.gif ) that it is insignificant.
Yet no-one has ever been able to detect these ‘matter waves’ or to explain what they might be.

Surely the De Broglie matter wave equation, is the mathematical equivalent of saying that every sub-atomic object has a magical talisman attached to it as do all objects in creation, the only catch is that  as objects become larger and reach the macroscopic scale the magical talisman can no longer be seen or detected.   To state that because of this ‘mathematically’  brilliant equation, light and all sub-atomic particles are subject to wave-particle duality is surely the overstatement of the century ? YET look at the picture of wave diffraction above and it is REALLY difficult to imagine that ALL solids such as dust, fine talc etc., that can be made to move in waves will not behave in a similar fashion undergoing diffraction in the right conditions!!!
« Last Edit: 31/07/2015 14:13:40 by McQueen »


 

Offline alancalverd

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Re: What is diffraction ?
« Reply #1 on: 31/07/2015 14:42:29 »
How large an object are you interested in?

We use neutron diffraction in crystallography, and AFAIK some significant buckyballs have been diffracted, but we rapidly approach the point at which the projectile momentum damages the diffracting edge.

You can indeed see diffraction and interference patterns in fluidised beds.   

The answer of course is that "wave particle duality" is a nonsense. We have two different and useful mathematical models of nature, neither of which is claimed to be complete or universally applicable.
 

Offline evan_au

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Re: What is diffraction ?
« Reply #2 on: 02/08/2015 00:45:54 »
Quote
possesses periodicity
I don't think that periodicity is a prerequisite for diffraction? However, a wave-like behavior is a prerequisite.

It is true that with periodic waves like the ocean waves in the photo above (plus sound waves, radio waves and laser light), diffraction patterns are easy to detect.
  • However, the diffraction pattern also appears with non-periodic functions like low-intensity/single-photon light, neutron beams, electron beams, and streams of buckballs. These patterns are harder to detect, because you have to wait longer for a sufficient statistical sample to accumulate.
  • So the diffraction pattern at atomic scale is due to a particle (photon, neutron, atom or whatever) interfering with itself, as if a single particle passes through both slits simultaneously.
  • The diffraction pattern at the level of sound waves, ocean waves and radio waves is due to zillions of particles (air molecules, water molecules, radio-frequency photons), each carrying the same wavelength and phase, interfering with other particles carrying the same coherent wave. 
So I see no conflict with de Boglie's equation.

The challenge is to get dust or talc to move as part of a coherent wave. One way I have seen to do this is to get a large metal sheet, suspended so the edges are free to move. Sprinkle the top with talc. Play the sides of the sheet with a violin bow, and you will see the talc move towards the vibration nodes (just don't do it with your favorite Stradivarius!). I am sure you could set up a dual-slit version of this experiment...
 

Offline Colin2B

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Re: What is diffraction ?
« Reply #3 on: 02/08/2015 16:30:36 »
I don't think that periodicity is a prerequisite for diffraction? However, a wave-like behavior is a prerequisite.
If you mean periodicity between particles I agree. Obviously wave like behaviour of particles eg photons implies an internal period or wavelength even for something we would consider a short burst of wave amplitude. Certainly the wavelength of sound and light has an effect on the degree to which it is diffracted.
As the OP indicates larger particles have very short wavelengths which would diffract less easily, and certainly would never pass through a slit of one wavelength. I think there was some surprise amongst the experimenters when some of the larger molecules showed a diffraction pattern.

  • So the diffraction pattern at atomic scale is due to a particle (photon, neutron, atom or whatever) interfering with itself, as if a single particle passes through both slits simultaneously.
'As if' is an important point here. You are right to separate the mathematical terminology (follows all paths etc) from the experimental evidence of individual photons superposing onto zones matching the calculated probability distribution. Darker areas being a lack of photons rather than a cancellation.

The challenge is to get dust or talc to move as part of a coherent wave ..... I am sure you could set up a dual-slit version of this experiment...
I think the talc particles are too large to go through a wavelength slit. However, I have seen sand particles showing diffraction patterns but I assume this is more due to oscillations in the wind strength.

One thing worth remembering in all of this is that although we talk of sea waves diffracting, many of the patterns are often due to refraction where shallowing sea bed near the edges of openings causes the waves to slow down so the central waves in deeper water catch up. This is the same effect which causes waves at an angle to the seashore to turn so they end up breaking parallel to the shoreline.[/list]

Edit: forgot to say:

The challenge is to get dust or talc to move as part of a coherent wave. One way I have seen to do this is to get a large metal sheet, suspended so the edges are free to move. Sprinkle the top with talc. Play the sides of the sheet with a violin bow, and you will see the talc move towards the vibration nodes (just don't do it with your favorite Stradivarius!). I am sure you could set up a dual-slit version of this experiment...
What you have seen are not difraction patterns but Chladni patterns. The plate vibrates and, as you say, the talc (I use glitter,)moves away from the antinodes and accumulates in the nodes. We use them in musical instrument making to examine the resonance modes of soundboards.
« Last Edit: 02/08/2015 22:59:02 by Colin2B »
 

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Re: What is diffraction ?
« Reply #3 on: 02/08/2015 16:30:36 »

 

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