Answer:
Too small to be seen with an optical microscope
Step-by-step explanation:
An electron microscope is a type of microscope that is used to observe very tiny specimens whose features can not be observed by other types of microscopes. It uses a beam of electrons to generate an image of a given specimen whose features can be clearly observed and studied.
Electron microscope has very high resolution and magnification as compared to other optical microscope hence can be observed in the study of micro-organisms such as viruses which would be difficult to study their features using optical microscopes.