Numerical Aperture: The One Number That Controls Your Resolution 🔬

Added: 2026-05-06

[00:00:00]There's one number on your objective that controls resolution. Last video we saw that resolution depends on [music] wavelength and numerical aperture or NA.

[00:00:09]NA equals N * the CANA of theta. N [music] is the refractive index of the medium between the objective and the specimen. Theta [music] is the half angle of the cone of light the objective collects. Wider cone, more light, [music] finer detail. In air, N is 1.0.

[00:00:25]So NA tops out around 0.95.

[00:00:28]Wide-angle rays bend away at the [music] glassto air interface and never reach the lens. Immersion objectives solve this with [music] oil, water, or glycerol between the lens and the cover slip. The higher [music] refractive index keeps those rays on course and into the objective. Not more magnification, [music] more light. You can't change physics, but you can pick the [music] right objective. An NA is the spec to check first, not magnification. Next, upright or inverted, two microscope designs, and your sample decides which