What does the numerical aperture of a lens indicate?

The numerical aperture (NA) of a lens is a fundamental characteristic that quantifies its ability to collect light and resolve fine specimen detail at a fixed object distance. It is calculated based on the refractive index of the medium in which the lens operates and the sine of the half-angle of the maximum cone of light that can enter or exit the lens. A high numerical aperture indicates that the lens can capture more light from the specimen and therefore produce a clearer and brighter image.

In practical terms, a lens with a higher numerical aperture will be able to resolve smaller structures and provide better contrast, playing a crucial role in microscopy and similar fields. Thus, the correct answer reflects the lens's capacity to gather light, which is essential for producing high-quality images in cytogenetics and other applications.