Fig. 5.1 — Comparison of optical forces calculated in various trapping regimes

Transverse trap stiffness produced by a 10 mW laser beam of wavelength λ₀ = 632 nm focused by a 1.20 NA objective on a dielectric sphere of radius a (n_p = 1.50) in water (n_m = 1.33). The solid line represents the exact electromagnetic calculation. The dipole approximation (dotted line) works for small spheres (a ≪ λ₀). The geometrical optics approximation (dashed line) works for large spheres (a ≫ λ₀).