Under a light microscope, which organisms can you see?

• A. Viruses.
• B. Bacteria bigger than 200 nm.
• C. Prions.
• D. Fungi.

Answer: (B)

Under a light microscope, you can clearly see objects that are larger than about 200 nanometers because of the diffraction limit of light. That threshold means anything bigger than ~200 nm can be resolved as a distinct object when proper staining and magnification are used.

Bacteria usually range from about 0.5 to 5 micrometers (500–5000 nm) in size, so they readily fall above the 200 nm limit and can be seen under standard light microscopy. Prions are extremely small proteins, and many viruses are also smaller than 200 nm, so they aren’t resolved as discrete particles with light microscopy and typically require electron microscopy. Fungi come in larger forms too (yeasts are a few micrometers across, hyphae are even larger), so they can be seen as well, but the size criterion most reliably distinguishes what’s routinely visible in basic light microscopy—bacteria bigger than 200 nm.