What are the limitations of a compound microscope?
In a light microscope, the light waves will spread out whenever they pass through a lens, or any sort of obstruction. The primary lens in most microscopes is of much bigger than 300-900 nanometres, so the angle by which light waves diverge is really quite small. However, it is not zero the light waves do spread out a little bit, and the result is that the visual field is always a bit blurry. It is impossible for an ordinary light microscope to avoid this problem, so they can never see structures smaller than about 500 nm.