What is the approximate resolution of a light microscope?

• A. Approximately 0.2 nm
• B. Approximately 2 nm
• C. Approximately 200 nm
• D. Approximately 20 nm

Answer: (A)

Light microscopes are limited by diffraction. The smallest detail they can separate is roughly d ≈ λ/(2NA). With visible light (about 400–700 nm) and a high-quality objective (NA around 1.3–1.4), this comes out to about 200 nm. So two points closer than ~200 nm can’t be resolved as separate features in a standard light microscope. Among the options, the one around 200 nm matches this limit best. The other values are far smaller than what a light microscope can normally resolve—0.2 nm is an atomic scale seen with electron-based methods, while 2 nm or 20 nm would require far more advanced imaging techniques not typical of conventional light microscopy.