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IrshadAlam
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I know the concept of diffraction of light, but not yet understood why light should bend passing the edges of objects ! I mean what makes it bend?
Cause of Diffraction of Light
4 posts - 4 authors - Last post: 24 seconds ago
To understand this you have to think of light as a wave and not a ray. ... each part of the wave front acts as a wave generator. ...
www.physicsforums.com/showthread.php?p=2900666
Idoubt said:To understand this you have to think of light as a wave and not a ray. For example suppose you have a tub of water and you place some sort of wooden surface vertically in it with a slit.
If you disturb the water you will see waves propagating in all directions on the surface.
When the waves hit the wood it will pass through the slit, but it won't emerge through the slit on the other side as a straight line, it will still spread in concentric circles ( well semi-circles ) on the other side of the surface with the slit as the centre.
IrshadAlam said:The water waves are made up of material where as light waves are electromagnetic waves. So that can't be example of diffraction.
IrshadAlam said:I have never said that water waves and sound waves don't diffract.
But their properties are different and the properties of light are very much different.
water waves diffract because of property of fluid that when they touch the edge of an object there is a phenomenon of adhesion with the edge of object and hence they bent towards the direction of object.
Regarding sound waves, the waves travel by striking the medium. And so when there is an object in their path they strike to that object also and the sound is scattered through every direction.
This can't be similar to light waves. Because light waves neither have adhesion properties nor they strike to the medium in their way.
There is only waves and photons traveling, when we say light is propagating.
In the Hyugen's double slit model, its only explained how the diffraction occurs and not WHY the waves bent while passing through an edge of an object.
There may be lack in my knowledge of laws but please try to understand my point.
Andy Resnick said:I think you are confusing mechanisms with properties.
Surface water waves are indeed caused by interfacial energy- that's why spreading oil on water abolishes surface waves. And sound waves are indeed caused by variations in the density of a compressible medium.
But diffraction is a property of waves- not the constituent particles. Diffraction is caused by the spatial truncation of a wavefront.
IrshadAlam said:That's not my query.
Please could anyone tell me that why should light bend towards a specific direction when it passes through an edge of an object?
cragar said:You could explain diffraction in terms of the uncertainty principle .
If we try to force light through a small opening, we know to well where its at so we get an uncertainty in its momentum and it starts to spread out .
Born2bwire said:I do not think that works since diffraction is an established macroscopic classical property. The properties that give rise to the Heisenberg Uncertainty Principle are not present in the physics or mathematics of classical wave physics.
jtbell said:In fact, they are. From Fourier analysis, a wave packet (whether classical or quantum-mechanical) must obey the relationship
[tex]\Delta x \Delta k \ge \frac{1}{2}[/tex]
In QM this becomes the HUP via [itex]p = \hbar k[/itex].
Born2bwire said:But this creates a problem because we experience diffraction with monochromatic plane waves.
Andy Resnick said:Be careful- we do not observe diffraction from a monochromatic *plane* wave- a plane wave extends to +/- infinity in the transverse direction. We *do* observe diffraction from a monochromatic wave that consists of a plane wave truncated by an aperture.
Born2bwire said:You do not need an aperture to diffract the wave. You can have a finite plate or wedge and still achieve diffraction.
[snip]
EDIT: Or perhaps you are misunderstanding my meaning.
Andy Resnick said:we could be misunderstanding each other. A plane wave (that is, a solution e^ikz or e^ikr) formally has infinite extent and so does not diffract. We agree that when this is not the case: aperture, scatterer, 1/2 plane screen, etc, diffraction occurs. Bessel beams are another example of a non-diffracting field, but one that has finite spatial extent.
sophiecentaur said:Confusing if you think in terms of the photons all being there all the time and moving through space. Not so confusing if you think in terms of waves and then say that the photons aren't really 'there' except when the interactions of source or detector occur.
IrshadAlam said:How can you assume photons aren't there, in fact photons play an important role in both refraction and diffraction
Born2bwire said:Since the wavelength in water is different from that of air, the wave has to bend so that the phases of the wave above and below the interface match up.
IrshadAlam said:But bending is not The only option with the wave when trying to match the phases above and below.
Sorry for limited discussion here, because I am working up on it and can't explain without proper results; but here I would say that relying on Hyugen's principle in diffraction an Snell's Law in refraction does not troubleshoot the queries that arise in the minds of youngsters because it is a basic physics and experienced everyday.
There is a great need to study on quantum physics so as to understand the role of photons in both phenomena refraction and diffraction of light. I THINK
Born2bwire said:What would you suggest instead of refraction to keep the phase continuity across the boundary? The refraction comes out naturally from the boundary conditions that arise with the vector wave equation. There actually isn't any need to introduce the physics into the derivation as it can be done intuitively with the math.
IrshadAlam said:I have not said there is other substitute to refraction but only for the reason that you said refraction would not occur.
we can also prove some unnatural and unreal things using mathematical equations, but in all that physics must not be ignored.
One example is that mathematically and with the help of figures I have a fake proof of exactly inverse of snells law and it says that velocity of light in water is higher than that of in air.(Just fake and I disbelieve it).
We will have to think sticking to physics and not mathematical equations.
Born2bwire said:Then what is your objections to the explanations given previously? To whit, we have mentioned Huygen's Principle and Equivalence Source Principle to explain diffraction. For refraction we can explain it by the fact that boundary conditions require that the phase of the wave be continuous across the interface in addition to the fact that the wavelength changes as well.
IrshadAlam said:Please give me the textual material of latest mathematical proofs that you are telling about or just send me their links then I will explain you about my objections. Please Believe
IrshadAlam said:Please give me the textual material of latest mathematical proofs that you are telling about or just send me their links then I will explain you about my objections. Please Believe
sophiecentaur said:These 'latest mathematical proofs' are pretty ancient and you can find them, as has been said, in so many established texts. That doesn't mean they're flawed, though. Explain your objection to those proofs first and show how introducing photons actually improves on them.
You will, of course, need to characterise the photon very fully, with regard to its extent, shape etc. and, possibly, how it will interact with other photons whilst it's going from A to B. You may find that a bit difficult.