Numerical aperture (NA) is a dimensionless number that characterizes the light-gathering ability of an optical system, such as a microscope objective or a lens. It is defined as the sine of the half-angle of the maximum cone of light that can enter or exit the lens, multiplied by the refractive index of the medium in which the lens is working. A higher NA indicates better resolution and the ability to capture more light. In microscopy, numerical aperture plays a crucial role in determining the clarity and detail of the images produced. For example, objectives with higher NA values are essential for observing fine structures in biological samples, as they can resolve smaller features. Understanding NA helps users select the appropriate lenses for their specific imaging needs.