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I’m not a physicist but a curious reader, and I’m trying to understand the slit experiment it all makes sense if I accept that the slit itself has no active affect ... can any one explain what actually happens in the slit, given that even if it is only one atom thick it still has a material presence? Really appreciate any help in understanding this Thank you Frances

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    $\begingroup$ Could you clarify what you mean by "it all makes sense if I accept that the slit itself has no active affect" $\endgroup$ – owjburnham Aug 13 at 11:30
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    $\begingroup$ You should really go see Veritasium's video ( youtu.be/a8FTr2qMutA ). He explains the Uncertainty Principle using slit diffraction. It can give you a fair idea of what is actually going on in the slit. The slit is not actively participating in the experiment and so it is not causing any fringe pattern. It is the wave nature of the photons as they diffract through the slit that is causing the fringes on the screen. In my opinion, You should also watch Eugene Khutoryansky's video on this ( youtu.be/iVpXrbZ4bnU ) to get a reasonably good analogy. $\endgroup$ – Shishir Maharana Aug 13 at 11:40
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    $\begingroup$ All the slit does is limit the possible wavelengths of the transverse momentum. The rest falls out from math. $\endgroup$ – Jon Custer Aug 13 at 13:02
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    $\begingroup$ I doubt that there is a classical description of what is happening at the slit. This means that you will necessarily have to accept a quantum mechanical explanation, whether or not that explanation seems to "make sense". $\endgroup$ – David White Aug 13 at 15:15
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    $\begingroup$ @DavidWhite if the question is about optical diffraction, it's explainable with classical electromagnetics. No QM required. $\endgroup$ – The Photon Aug 13 at 16:29
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The best way to imagine it without getting too deep into mathematics is as Solomon's comment suggests. Imagine a pond with a wall through the middle, and imagine that the wall has a slit in the middle. If there are waves with wavefronts parallel to the wall hitting the wall from one side, some of the wave will get through and form a more circular wave.

enter image description here

In the slit experiment, of course, light waves are used in stead of water waves, so the behaviour will be slightly different. For example the width of the slit will determine which wavelengths of light will be diffracted. Say the slit has width $d$, then light with wavelength $\lambda\approx d$ will be diffracted. This can also be illustrated in a pond:

enter image description here

If the slit is larger than the wavelength, the waves will behave as in the 'large gap' image above. As the gap becomes larger, the wavefronts to the right of the gap will become closer and closer to being parallel to the wavefronts left of the gap (an thus not being diffracted). So in the case of light, a slit of only one atom thick will show its presence when light with wavelength around the size of an atom ($\sim10^2\text{pm}$) is sent through the slit it will be diffracted.

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A photon is a wave in the EM field that has defined electric and magnetic components.
Photons can also be scattered (direction change) by an interaction with electrons in matter and this is the main reason for diffraction. (Also photons are only created by electrons (mostly in atoms) and are only absorbed by electrons in atoms).

The interaction of photons with matter is governed by Maxwell’s and Fresnel’s equations, which govern the reflection, transmission, and deflection of light rays using the magnetic and electric susceptibility factors of any given material and the EM variables of the light.
Which photons and how strong they interact as they pass through the aperture is also due to probability (Quantum Mechanics) or, when there are many photons, they can be modelled classically (on average).
Any aperture (even a hula-hoop) will effect light passing through it, and you can see wiki about the Airy disk for a circular aperture.

What is interesting about an aperture is not only this “scattering” of light but the fact that “interference” patterns are also formed when the photons are viewed on a screen. This “interference” can be viewed with older classical theory as photons cancelling each other in the dark areas ( but violation of energy conservation) or can be viewed quantum mechanically (wave function of light) where the dark areas have been shown to be where no photons fall.
In the QM “photon wave function” explanation light must travel a path that is a multiple of its wavelength (Feynman), thus certain paths are not possible (dark areas - no photons) and the bright areas contain all the photons. This is similar to how light behaves in a laser cavity for example.

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