= Optical momentum efficiency
{title2=$F=QP/c$}
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 $Q\simeq2\mathcal R$ in the simple vacuum-momentum convention.
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