Wave propagation in disordered media such as light propagation in a cloud can be roughly described by diffusive transport of the wave intensity. This description which is well known from the Drude model for electronic transport totally ignores the wave nature and hence fails to describe interference effects such as optical speckles which are fingerprints of the disorder. The particular interference of waves traveling along reciprocal multiple scattering paths is always constructive in the backscattering direction, even after averaging over many speckles, and thus gives rise to enhanced backscattering and (weak) wave localization. P.W. Anderson predicted in 1958 a breakdown of the wave transport on macroscopic length scales which should occur when the scattering strength of the medium becomes very high, i.e. when the transport mean free path $l$* becomes smaller than the wavelength $2pi /k$. I shall discuss recent experiments [1-5] which provide first clear evidence for the transition from weak to strong localization of visible light and allow a quantitative analysis of the localization transition in terms of the scattering strength $1/kl$* and residual absorption. par noindent [1] Martin Störzer, Peter Gross, Christof Aegerter and Georg Maret, Observation of the critical regime near Anderson localization of light, Phys.Rev.Lett. 96, 063904 (2006) par noindent [2] C.M. Aegerter, M. Störzer and G. Maret, Experimental determination of critical exponents in Anderson localization of light, Europhys. Lett. 75, 562 (2006) par noindent [3] M.Störzer, Ch.M.Aegerter, and G.Maret, Reduced transport velocity of multiply scattered light due to resonant scattering, Phys. Rev. E 73, 065602(R) (2006) par noindent [4] P. Gross, M. Störzer, S. Fiebig, M. Clausen, G. Maret, and C.M. Aegerter. A precise method to determine the angular distribution of backscattered light to high angles. Rev. Sci. Instrum. to be published (2007) par noindent [5] C.M.Aegerter, M.Störzer, W.Bührer, S.Fiebig and G.Maret, Experimental signatures of Anderson localization of light in three dimensions, J.Mod.Opt. submitted (2007)

Fachbereich Physik, Universität Konstanz, Germany