# Is there an atomic level explanation for diffraction of light around opaque aperture?

It has always been a hard time understanding the phenomenon of diffraction of light around opaque objects. Hoe does it happen? Why does transparent objects do not diffract light? What quantum mechanics operate that bends light? I used to have an intuitive understanding that surface atoms of the aperture edges reflect the light that interferes and forms the fringes.

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The answer to this can be found in Huygen's Principle, suggested by him in the 1600's and put on firm theoretical ground in the 1800's by Fresnel and Kirchoff.

Waves spread because each point on any surface (a mathematical surface in space, not a surface of a real material) can be considered to be a point source of new radiation. Each point generates a spherical wavelet. In the absence of obstacles, interference of all these wavelets assures that the sum generated from a plane wave is again a plane wave, and the plane wave propagates as a plane wave. With an obstacle in the way, the wavelets radiate into the shadow of the object: diffraction.

Wikipedia has some good simulations, and I'm sure there are many others.

Transparent objects do diffract light, but we usually have a different name for the phenomenon: refraction. Instead of blocking the light and stopping the radiation from a set of wavelets, a transparent medium changes the phase of the wavelet as the disturbance propagates through the medium. Once the radiation has passed through the medium and has emerged, we add up the radiation from all the wavelets, those that pass by the object and those that pass through it, to get the complete field. In terms of Huygen's Principle, diffraction and refraction have the same explanation, and are two manifestations of the same effect.

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This has nothing to do with quantum mechanics. The underlying physics is no more complicated than that of water waves hitting a stationary pillar or rock (or being able to have a conversation with someone around the corner of a building).

Waves spread out. and when the peak of a wave intersects with the trough of another wave, they cancel. When the peaks/troughs coincide, they add. So, when a wave is blocked, it causes the waves that make it around the edges to spread out from the boundary and recombine.

I'm sure someone else will have a fancier, more complete answer than this, complete with pictures. But this is the essence of it.

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Your classical explanation is right, but to say "this has nothing to do with quantum mechanics" is misleading, don't you think? – user35033 May 15 '14 at 19:15
@user35033: invoking quantum mechanics just confuses the underlying principle. In what way is simple diffraction like this different from the diffraction of water waves? – Jerry Schirmer May 15 '14 at 19:25
The fact that light is made of photons is the primary difference. Diffraction is completely quantum mechanical. Light behaves like a classical wave, but that's just convenient, and not indicative of the underlying mechanism. – user35033 May 16 '14 at 2:57
@user35033: there is no quantum behavior in the bulk. Photon number isn't even a good quantum number of a laser state, if you want to go there. Just go back to Jackson. Light diffraction has nothing to do with quantum mechanics. – Jerry Schirmer May 16 '14 at 3:14
The classical explanation for the diffraction is unconvincing in many respects. Firstly, in the case of water waves it is the water molecules that move collectively up and down transferring the wave through its surface tension/molecular cohesive forces. similarly for a longitudinal wave of a rope tied to a wall operates with the tension forces in the member. By what principle does the magnetic fields transfer the fields/energy? The fact that many theories to understand the light have all been different hypothesis without any consensus makes it even more difficult to contemplate the phenomenon. – Rams May 16 '14 at 13:57

Rams, I fully agree with your thoughts about the influence of atoms to diffraction. To use it as a alternative explanation for the interference phenomena is overdue. Quantum mechanics is talking about the quantization of fields but do not associate this fact due to the interaction between photons and the electric potential of the surface electrons of the edges.

As you mentioned in your comment there are substantial differences between water waves and electromagnetic waves. In my own understanding photons are corpuscles with alternating electrical and magnetic fields and dependent on the distance of the photon from the edge and its current electromagnetic state they form a quantized field with the edges.

That happens with everey single photon. And this is an easier explanation for single photon experiments. No interference with itself. Everybody who learned it in the common way is satisfied with the existing formulas (and this formulas are working well) and this is ok. But I'm sure that there will be in the future better formulas including the common field generated between the surface electrons of the edges and the photons.

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