 Optical momentum efficiency

An optical momentum efficiency $Q$ expresses the net axial momentum transferred to a bead relative to incident power $P$ divided by vacuum light speed. It absorbs medium-index factors and the incident/outgoing ray geometry. Refractive-index contrast alone does not determine $Q$ for a transparent bead illuminated by a focused beam. The estimate $Q\sim m-1$ is a weak-contrast scale, not a universal force formula. An effective reflector with power reflectance $\mathcal{R}$ instead gives $Q\simeq 2\mathcal{R}$ in the simple vacuum-momentum convention.