Numerical aperture

What is Numerical aperture?

Numerical aperture definition and meaning on Dictionary terms:
noun Microscopy.
a measure of the resolving power of a microscope, equal to the index of refraction of the medium in which the object is placed multiplied by the sine of the angle made with the axis by the most oblique ray entering the instrument, the resolving power increasing as the product increases. Abbreviation: N.A.