Numerical Aperture (NA) is a dimensionless number that characterizes the range of angles over which a microscope objective can accept or emit light. It is crucial for determining the resolving power of optical systems, such as microscopes and fiber optics. A higher NA indicates a greater ability to gather light and resolve fine details in a specimen. The formula for calculating NA is NA = n × sin(θ), where n is the refractive index of the medium between the lens and the specimen, and θ is the half-angle of the maximum cone of light that can enter or exit the lens. Understanding NA helps in selecting the appropriate optical components for various applications.