  There are two different proposals for the momentum of light in a transparent
dielectric of refractive index n: Minkowski's version nE/c and Abrahm's version
E/(nc), where E and c are the energy and vacuum speed of light, respectively.
Despite many tests and debates over nearly a century, momentum of light in a
transparent dielectric remains controversial. In this Letter, we report a
direct observation of the inward push force on the end face of a free nm fiber
taper exerted by the outgoing light. Our results clearly support Abraham
momentum. Our experiment also indicates an inward surface pressure on a
dielectric exerted by the incident light, different from the commonly
recognized pressure due to the specular reflection. Such an inward surface
pressure by the incident light may be useful for precise design of the
laser-induced inertially-confined fusion.
