Organisms that are smaller than the diffraction limit of light cannot be viewed using a compound microscope. The diffraction limit is the minimum size of an object that can be resolved by a microscope, and it is determined by the wavelength of light used and the numerical aperture of the objective lens. The diffraction limit for visible light is about 200 nanometers (nm), which means that objects smaller than this cannot be seen with a compound microscope. Many bacteria, viruses, and other microorganisms are smaller than 200 nm, so they cannot be viewed with a compound microscope.