  We study the Brownian motion of microbeads immersed in water and in a
viscoelastic wormlike micelles solution by optical trapping interferometry and
diffusing wave spectroscopy. Through the mean-square displacement obtained from
both techniques, we deduce the mechanical properties of the fluids at high
frequencies by explicitly accounting for inertia effects of the particle and
the surrounding fluid at short time scales. For wormlike micelle solutions, we
recover the 3/4 scaling exponent for the loss modulus over two decades in
frequency as predicted by the theory for semiflexible polymers.
