Final answer:
Electron microscopy, employing devices such as TEM and SEM, allows for the visualization of virions which are too small for light microscopes. However, it doesn't typically distinguish infective from non-infective virions. Infectivity assessments require additional methods.
Step-by-step explanation:
Electron microscopy is indeed useful for counting viruses as it allows scientists to see objects that are much smaller than what can be resolved by light microscopes. Electron microscopes like the Transmission Electron Microscope (TEM) and Scanning Electron Microscope (SEM) are capable of visualizing virions, which are typically between 20 and 250 nanometers in diameter, and therefore much too small to be seen with a light microscope. However, electron microscopy is not typically used to distinguish between infective and non-infective virions; this would generally involve additional biochemical or molecular techniques to determine the infectivity of the virus particles.