Diffraction, at least as its commonly explained, happens when light crosses an obstacle "of size in the same order as its wavelength". For example, a small slit. However, in one of my labs, we used ~0,2 mm slits; while using light in the order of 650 nm. That's a difference of 3 orders in magnitude; yet diffraction still happened as expected.
Is the simplistic definition of diffraction wrong? Or is the margin larger than the same order of magnitude?