A silica bead of refractive index is immersed in water of refractive index and held…
PHYSICS · Multi-part · Fermi University Physics · Biophysics & Medical Physics · Molecular machines, transport and bioenergetics · Molecular machines, transport and bioenergetics
Forces, Machines and Signals, Ch Measuring one molecule: traps, tweezers and single-molecule fluorescence
Problem
A silica bead of refractive index n1=1.45 is immersed in water of refractive index n2=1.33 and held near the focus of a near-infrared laser.
((a)) Compute the relative refractive index m=n1/n2 and the Clausius-Mossotti sign factor (m2−1)/(m2+2).
((b)) State, with a one-sentence reason, whether the bead is pulled towards the focus or pushed away from it, and explain why stable three-dimensional trapping also requires a high-numerical-aperture objective rather than radiation pressure alone.