Numerical aperture
= Numerical aperture
{title2=$\mathrm{NA}=n\sin\alpha$}
{wiki}
The <numerical aperture> is $n\sin\alpha$, where $n$ is the <refractive index> of the medium between object and objective, and $\alpha$ is the half-<angle> of the admitted cone of <light>. Specify object-side or image-side <numerical aperture>; the microscope convention usually means the object side.<Numerical Aperture by Michael W. Davidson>{ref}