Final answer:
Viruses are generally too small to be seen with light microscopy and require electron microscopes for visualization. The virions' tiny size, typically 20-250 nm, became observable only after the development of electron microscopy.
Step-by-step explanation:
Most viruses are ultramicroscopic, with virions (individual virus particles) ranging from about 20 to 250 nanometers in diameter, and thus are not visible under light microscopy.
Some of the largest viruses, such as Pandoravirus salinus and Pithovirus sibericum, can reach up to 1000 nm and are comparable to the size of small bacterial cells. To visualize viruses and obtain detailed information about their structure, scientists must use electron microscopy, including both scanning and transmission electron microscopes.
This technology grants us a clearer understanding of the surface and internal structures of virions, which are otherwise not resolvable with light microscopes. It was after the invention of the electron microscope in the late 1930s that detailed studies of viruses like the tobacco mosaic virus were made possible.