  Using a known distribution of the Meissner currents over the surface of an
infinitely long superconducting slab with a rectangular cross section, we find
an applied magnetic field at which vortices begin to penetrate into the
superconductor. This vortex-penetration field is determined by an interplay of
the geometrical and Bean-Livingston barriers. The obtained results enable one
to find the lower critical field and the London penetration depth from
measurements of the magnetic induction on the surface of the superconducting
slab, using, e.g., the micro-Hall probes.
