What is the Fundamental Reason for EMR Diffraction?

[ChrisXenon]

I know the textbook definitions and descriptions of the phenomenon, but I'm hoping for a fundamental WHY.

I can see that when a water wave passes an obstacle, the wave spreads out into that object's shadow because the wave's energy is not constrained to any direction and so it will move out in all directions; it is essentially a "pile of water" being pulled down by gravity and it'll be pulled down in all directions so each part of a water wave is like a point source; when part of a wave front, complex interactions occur between all water piles and the emergent result in a wave front. But when the wave front is disrupted by the obstacle the water piles on the end are free to slump in all directions away from the wave front - so the shadow is filled by circular waves propagating from the edge of the wave front. Fair enough. But what is the mechanism which leads to the analogous behaviour with light/photons?

 

[sophiecentaur]

but I'm hoping for a fundamental WHY.

There is never a good answer to the "Why?" question. Physics never attempts one. All you can hope for is the description of a process that's based on relationships between some basic quantities; i.e. what you can expect to happen under certain conditions. This is not a cop-out answer. Go where you like and you will not get a satisfactory answer to the "Why" question from anyone. This is a hot topic on PF and elsewhere in 'legit' Science discussions. Google "Richard Feynman Why question" and listen to what he has to say. Few people will attempt to contradict that iconic figure'; he has defined the terms.

But what is the mechanism which leads to the analogous behaviour with light/photons?

You want a wave description so you can only get a wave description; photons don't come into it. Photons are seldom the best approach to non-quantum, problems.
You can rely on what the Maths tells you about this sort of thing. Maths is a descriptive language that carries much more 'real information' about a process than any amount of comforting 'physical' verbal descriptions.
If you take the Wave Equation (A second order differential equation in space and time) for an EM wave and you impose boundary conditions - a reflector / a boundary between media with different wave speeds etc. etc. the resulting solution to the equation gives you all the familiar diffraction and refraction phenomena.
For a good metallic reflector, there are currents induced in the surface by the incident wave and those currents set up a new, reflected wave which (when the region of reflection is many wavelengths wide) follows the classical laws of light reflection and also gives the expected diffraction pattern for small objects.

 

[ChrisXenon]

There is never a good answer to the "Why?" question. Physics never attempts one. All you can hope for is the description of a process that's based on relationships between some basic quantities; i.e. what you can expect to happen under certain conditions. This is not a cop-out answer.

Hmm. I can see why ultimate WHYs are like ultimate TRUTH and can't be obtained. However a "why from the level below" would satisfy me. We routinely use models and analogies to facilitate what we call "understanding" and those help us advance in science & engineering in ways that suit us. Certainly those models and analogies are not ultimately true but they are true enough to be useful. That's what I'm after for diffraction.

Thanks for your help anyway.

 

[davenn]

But what is the mechanism which leads to the analogous behaviour with light/photons?

As Sophicentaur said ... photons are irrelevant ... there are no issues explaining it with classical physics
Photons are just quantum packets of energy

We routinely use models and analogies to facilitate what we call "understanding" and those help us advance in science & engineering in ways that suit us. Certainly those models and analogies are not ultimately true but they are true enough to be useful. That's what I'm after for diffraction.

Reflection, refraction and diffraction are very well understood for a long time using classical physics ... read any decent textbook
You seem to be looking for/expecting something that doesn't exist

 

[tech99]

Diffraction is a complex issue and Huyghens did not seem to entirely solve it with his secondary wavelets. For instance, if you are in the shadow of an object and you look at the edges of the object they are bright. Further, we do not see an interference pattern within the shadow, arising from the secondary sources.Thomas Young proposed in 1802 that the diffracted wave originates at the edge of the obstruction. It helped to explain the bright edges of an obstacle and why a sphere and a disc produce a bright dot in the centre of the shadow but with different intensity. It also explained why we do not find interference fringes behind an object. Young's idea has been gradually refined up to the present and it is now used a lot in antenna engineering. Whilst no one can say Maxwell's Equations are incorrect, I understand that they did not easily lead to a full explanation of diffraction phenomena. One of the interesting issues is the presence of surface waves (creeping waves) on the obstacle.

 

[sophiecentaur]

One of the interesting issues is the presence of surface waves (creeping waves) on the obstacle.

There's no end to how much the noise performance of a microwave dish can be improved by controlling the currents that flow around and over the edges. The 'hot' Earth where the dish stands can contribute significantly to performance of a high performance radio telescope or link antenna.

 

[taverngeek]

I think the good answer to the "why" question is that is what the mathematics say will happen and is matched by experiments. That a lot of physics (and mathematics) are contrary to our intuitive understanding of the physical world. So it becomes easier to "understand" by following the mathematics which says there should be EMR diffraction.

 

[sophiecentaur]

I think the good answer to the "why" question is that is what the mathematics say will
happen and is matched by experiments. That a lot of physics (and mathematics) are contrary to our intuitive understanding of the physical world. So it becomes easier to "understand" by following the mathematics which says there should be EMR diffraction.

+1
Shying away from the Maths is very problematic because there is no 'obvious' tangible / mechanical explanation of diffraction phenomena. All I can say is that, once I bit the bullet and went through some very basic ideas about diffraction (required in A level courses even) the topic began to be 'intuitive' to me. We aren't actually born with any of these Physics Concepts; we need to learn each of them before our brains can intuit them.

 

[jartsa]

I can see that when a water wave passes an obstacle, the wave spreads out into that object's shadow because the wave's energy is not constrained to any direction and so it will move out in all directions; it is essentially a "pile of water" being pulled down by gravity and it'll be pulled down in all directions so each part of a water wave is like a point source; when part of a wave front, complex interactions occur between all water piles and the emergent result in a wave front. But when the wave front is disrupted by the obstacle the water piles on the end are free to slump in all directions away from the wave front - so the shadow is filled by circular waves propagating from the edge of the wave front. Fair enough. But what is the mechanism which leads to the analogous behaviour with light/photons?

Same mechanism?

"Piled up water" needs to be changed to something that corresponds to piled up water in electro-magnetic waves. "Superposed electric fields" seems appropriate to me.

 

[A.T.]

I can see that when a water wave passes an obstacle, ... But what is the mechanism which leads to the analogous behaviour with light/photons?

If you understand it for waves, why not stick with the EM-wave model of light?

Are photons really more intuitive to you, including their quantum properties like the uncertainty principle? What does the uncertainty principle say about the momentum of a photon that you localize spatially to be exactly at the edge of an obstacle?

 

[ChrisXenon]

Same mechanism?

"Piled up water" needs to be changed to something that corresponds to piled up water in electro-magnetic waves. "Superposed electric fields" seems appropriate to me.

Jartza - thanks I think you hit the nail on its head. The "piled up water" relies on gravity to provide the reason for it to "slump" in all directions; to be the new point source; I don't know if a super-position of electric fields provides the same answer in EMR land though.

 

[tech99]

Jartza - thanks I think you hit the nail on its head. The "piled up water" relies on gravity to provide the reason for it to "slump" in all directions; to be the new point source; I don't know if a super-position of electric fields provides the same answer in EMR land though.

I think the "piled up water" idea is roughly akin to Huyghen's wavelets. Alternatively, the obstruction is a source of re-radiation.