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mwengler comments on If You Were Brilliant When You Were Ten... - Less Wrong Discussion
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This is not one I observed myself, but it circulated when I was around.
Feynman was sitting on the qualifying exam of a theoretical physics PhD candidate. If you have read Feynman you know he always realated theory to reality. So he asked this student "what is the wavelength of visible light?" The student said he had no idea. As is common in an exam like this, Feynman invited the student to consider what he knew about the real world and see if he could come up with something. The student suggested "the wavelength is 1 in normalized units" which is just pure BS anyway. Feyman asked the student to show him what he thought the wavelength might be by holding up his fingers. The student indicated with his thumb and forefinger about an inch apart.
Here's the punchline: Feynman says to the student and says "Do I look fuzzy to you?"
I found this hilariously funny when I first heard it, which was after I had been working on millimeter-wave quasioptical systems. I'll come back tomorrow and 'splain the punchline for those who don't care to learn quasioptics between now and then.
Well, at this point in the exam everything must have looked fuzzy to the hapless candidate. Seriously, though, it is hard to believe that someone at that level would not know basic stuff like that.
The wavelength of light must be smaller than something you can see using light. So the student should have known the wavelength of light must be smaller than the width of the hairs on his knuckles, < ~50 um. While this might seem to be a very specialized knowledge, the wave mechanics behind this limit feed right in to things like the heisenberg uncertainty principle and many of the properties of fourier transforms, really very basic stuff in lots of physics. If light had a wavelength of 1 inch, the best images we could make of the people we saw would have about an inch of "blurring" on them, looking like an out of focus picture.
The wavelengths and frequencies of all the sections of the em waves were required knowledge in my high school physics course.
Besides, these days everyone has a laser, and you can remember its wavelength. (My green one is 531 nm.)
"Actually, now that you mention it, I think I might need some new eyeglasses..."