Jean-Paul Kneib

title: Cluster Lenses - Nature's Telescope

abstract: 
Cluster of galaxies are now well known to display strong gravitational
lensing effects: giant arcs, multiple images and arclets. The lensing 
distortion/magnification is a function of the detailled mass distribution,
of the cluster, the distance of the background objects and to a lesser 
extend on the cosmological paramters through the angular diameter distance 
relations.  I will first present what is the current understanding of the 
mass distribution in cluster cores. From the mass distribution, we can then 
compute the distortion and magnification of these natural lenses, which 
properties are used to probe the distant Universe thanks to the
gravitational magnification. In particular I will present new observations 
of cluster cores in MIR and Submm, which shade new light in the properties 
of distant galaxies.