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RichardKennaway comments on Harry Potter and the Methods of Rationality discussion thread, part 28, chapter 99-101 - Less Wrong
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I have also read it, I don't remember where. It's not a particularly outré piece of knowledge, just a piece of knife-sharpening lore. The reason a properly sharpened blade does not show a visible edge is that the edge is thinner than the wavelength of light.
I do not know if this is true, which puts me in much the same position as Harry. It's just something I've read but never put to practical test.
As an owner of a penknife that I occasionally sharpen: It is definitely true that a dull blade gleams, but I am not sure if it is true that a sharp blade does not.
Sharp edges don't reflect any light visible to the naked eye, but it'll show up on an illuminated microscope.
The edge width is on par with the wavelength of light at ~300-500 nanometers at best