Light carries both energy and momentum; the energy of a single photon is given by its frequency times Planck's constant, but what is the momentum of light? Surprisingly [1], this simple science question has not been settled for over a century [2,3]. The problem occurs for light propagating in transparent media such as glass or water where the speed of light is reduced by the refractive index. Experimental tests [4-10] of the rivalling theories are rare and most seem to support the Minkowski momentum [4-8]. Here we provide the first quantitative experimental evidence for the Abraham momentum [3]. 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 theory based on the Abraham momentum. To reconcile the conflicting experiments, we argue that the Minkowski momentum matters in the wave aspects of light, whereas the Abraham momentum appears in the particle aspects. Apart from shedding light on a century-old theoretical problem, our findings are applicable in modern optomechanics where nanomechanical devices are driven, cooled and controlled via the momentum exchange between light and matter, and are relevant wherever light in media directly exerts momentum.

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