![]() The most dramatic example is inertial confinement fusion. Only relatively recently the momentum of light has been put to work in earnest. ![]() However, although the ratio between p M and p A is significant, the magnitude of the momentum does normally not amount to much: in free space 1 Ws of light carries a momentum of. In empty space, where n = 1, the momenta of equations ( 1) and ( 2) agree, but in media they differ by the square of the refractive index n, which amounts to a significant number (in glass ). Throughout this paper we regard n as independent of frequency within the relevant spectral range of the light. Abraham's formula, equation ( 2), describes the momentum p = mv of a particle of velocity and dynamical mass m where the mass is related to the energy by Einstein's. The Minkowski momentum, equation ( 1), follows from de–Broglie's relation for the momentum p of waves with wavelength and velocity. Minkowski and Abraham derived their formulae for the momentum of light within the classical theory of electromagnetic fields, but the two rivalling theories are most easily deduced from quantum mechanics and relativity. Apart from shedding light on a century–old theoretical problem, our findings are applicable in optofluidics that combines microfluidics with optics, and in optomechanics where nanomechanical devices are driven, cooled and controlled via the momentum exchange between light and matter, and are relevant wherever light in media exerts momentum. We found that if the light is not able to put the fluid into motion, Minkowski's momentum becomes effective, but if the fluid moves, it is Abraham's. To reconcile the conflicting experiments, we studied theoretically how the momentum transfer emerges in the interplay between optics and fluid mechanics. Sending light onto the surface of a liquid we have observed that the surface acts as a focusing mirror on the reflected light, in quantitative agreement with radiation pressure caused by the Abraham momentum.
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