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.