The best resolution of a confocal microscope using 500 nm light is 180 nm, determined by the confocal-specific Abbe diffraction limit formula. This value assumes optimal conditions with a high numerical aperture (NA) objective, typical for such exam questions. Confocal microscopy improves resolution over widefield by rejecting out-of-focus light via a pinhole.​

Resolution Formula

Lateral resolution in confocal microscopy follows $d=0.4\lambda /\mathrm{NA}$ or the full-width half-maximum (FWHM) approximation $0.51\lambda /\mathrm{NA}$, where $\lambda =500$ nm. High-end oil-immersion objectives achieve NA ≈ 1.4, yielding $d\approx 0.51\times 500/1.4\approx 182$ nm, rounded to 180 nm in standard references and CSIR NET contexts. Axial resolution is poorer at ~500 nm but the question specifies “best resolution,” referring to superior lateral XY resolution.​

Step-by-Step Calculation

Start with Abbe’s limit for widefield: $d=0.61\lambda /\mathrm{NA}\approx 0.61\times 500/1.4\approx 218$ nm. Confocal pinhole (at ~1 Airy unit) enhances this by √2 factor (~1.4×), reducing to ~0.43 λ / NA or 0.51 λ / NA for FWHM, giving 180 nm. For 500 nm green light and NA=1.4, sources confirm ~180 nm lateral as the practical best.​

Option Analysis (Typical MCQ)

CSIR NET-style options likely include 125 nm, 180 nm, 250 nm, 500 nm (based on exam patterns).​

• 125 nm: Super-resolution territory (STED/SIM), beyond confocal diffraction limit.​

• 180 nm: Correct; confocal optimal lateral resolution.​

• 250 nm: Widefield Abbe limit or lower NA (~1.0).​

• 500 nm: Wavelength itself or axial resolution.​

CSIR NET Application

This question tests diffraction limit understanding in confocal vs. widefield microscopy, key for life sciences syllabus on imaging techniques. Use blue-shifted light or higher NA for sub-180 nm in practice, but 180 nm is the textbook confocal benchmark for 500 nm.​