  Thermal motion gives rise to fluctuations in free surfaces; the propagation
of the thermally excited waves on such surfaces depends on the mechanical
properties of the medium. Their measurement can therefore provide information
on those properties. We have developed an optical tool to probe the thermally
excited waves on free surfaces: Surface Fluctuation Specular Reflection (SFSR)
spectroscopy. It consists in measuring the fluctuations in the position of a
laser beam, which is specularly reflected onto the free surface of a medium,
and is therefore sensitive to the roughness of that surface. We show how the
measured signal is related to the medium properties. We also present
measurements performed on Newtonian liquids as well as on a viscoelastic solid;
we show that, in all cases, there is a very good agreement between experimental
and computed spectra. SFSR thus applies to a broad range of materials. It
moreover offers a very good temporal resolution and should provide a useful
tool for dynamical measurements on complex fluids.
