Recently, the observation of low-power laser deformation of an air-water horizontal interface under total internal reflection (TIR) was argued [1]. This effect was claimed to be independent of the incident light-beam power and the interfacial tension and explained by a model that (i) accounts for the existence of a nonzero in-plane component of the optical radiation pressure exerted on the airwater interface, which was ascribed to the Goos-Hänchen spatial shift-a characteristic feature of TIR-and (ii) does not account for the force balance at the air-water interface. This Comment aims at correcting these two points. First, the optical force acting on the interface is always normal to the interface, be the situation associated to TIR or not. To prove it, we assume the fluids i ¼ ð1; 2Þ to be nonmagnetic, dielectric, transparent, and isotropic. Then, considering steady-state and time-averaged effects over an optical cycle, for the electromagnetic stress tensor (whose divergence gives the associated force density) is [2]
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