I Opaque-wall-with-hole argument from Feynman lectures

AI Thread Summary
The discussion centers on a concept from Feynman's lectures regarding the electric field on the opposite side of an opaque wall with holes. The main confusion arises from understanding how the electric fields at the wall (Ewall) and at point P (E'wall) can be equal despite the presence of holes, particularly when the holes are large. Participants note that the approximation holds better for larger holes, as smaller holes comparable to the wavelength may not yield the same results. The reference to Babinet's Principle is mentioned as a relevant concept that may clarify the argument. The discussion emphasizes the need for a deeper understanding of the conditions under which Ewall equals E'wall.
euphoricrhino
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In Feynman lectures vol I, last part of chapter 31, there was this argument about electric field on the other side of the opaque wall with holes.
The argument is attached below. I'm having a hard time understanding the claim in the red box. In particular, I failed to see how "fields arrive at the wall" not being changed by the plugs can be an argument for Ewall=E'wall, which are the fields generated by the wall charges at point P.
This argument seems to say that the approximation in the claim works well with "big" holes (not holes with size comparable to the wavelength). And the result was also used in the previous chapter, where the hole is pretty much half of the infinite plane. I'm failing to see when the hole is this big, Ewall=E'wall can still hold.

Did I miss something obvious? Can someone please enlighten me?
Thanks!
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