We have observed the propagation of spin waves and spin-wave packets in 20-nm-thick Permalloy films upon microwave excitation by means of
time-resolved scanning Kerr microscopy. Spin waves can be detected up
to 80 mu m away from the driving coplanar strip line. In doing so, we
were able to directly determine the phase velocities, dispersion
relations and group velocities for different geometries of an
externally applied in-plane magnetic field. In addition, we report on
the direct optical observation of spin-wave interference with lateral
and time (phase) resolution.