How small a detail can a compound light microscope resolve?

The resolution of a compound light microscope—the smallest distance at which two points can be distinguished as separate—is fundamentally limited by the physics of light and the numerical aperture (N.A.) of the objective lens.12

While magnification can be increased by using more powerful eyepieces, doing so beyond the limits of the objective's resolution results in "empty magnification," where the image is larger but no additional detail is revealed. The ability to resolve fine detail depends on the objective's capacity to collect diffracted light; generally, the best optical microscopes can resolve structures down to approximately 0.2 micrometres (μm).23

To achieve this level of resolution, researchers often use immersion objectives, which utilize a liquid medium between the specimen and the lens to increase the effective numerical aperture. By reducing the effective wavelength of light, these techniques allow for the observation of finer details than are possible with dry objectives.12