What type of organism is not visible with an ordinary light microscope?

Viruses are not visible with an ordinary light microscope due to their extremely small size, typically ranging from 20 to 300 nanometers. Light microscopes utilize visible light and optical lenses to magnify objects, but they lack the resolution needed to view structures at the nanoscale. In contrast, bacteria, fungi, and protozoa can be observed with light microscopy because they have larger cellular structures, generally in the range of micrometers. Therefore, viruses require more advanced microscopy techniques, such as electron microscopy, to be visualized clearly. This distinction highlights the limitations of light microscopes when it comes to studying smaller entities like viruses.