You&#x27;ll only get diffraction when the slits are comparable to the wavelength of the light, i.e., on the order of a micron.  I believe you can see light peeking out through a micron slit if you&#x27;re looking directly at it in a dark room, but I don&#x27;t think it&#x27;s feasible to see diffraction in shadows. The fuzziness you see in normal shadows comes from the finite angular size of the light source, not the wave nature of light.