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.
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 99, 100, and 101. The previous thread is at nearly 500 comments.
There is now a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system.
Also: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: