Philip H. Jones, Onofrio M. Maragò & Giovanni Volpe
Fig. 2.10 — Focusing a paraxial light beam
A paraxial light beam is split into a set of parallel light rays {rp(m)}. The focusing objective includes an aperture stop, or iris, with radius R and a focusing lens with focal length f, which bends the rays towards the focus O, obtaining the set of rays {rf(m)}. Here, we have a perfect focus without aberrations, i.e., all rays converge to O. The scattering of {rf(m)} produces a force FGO on the sphere whose centre is placed at C. The NA is related to the angle θ_max over which the rays are focused [Eq. (2.22)].