  Electromagnetic vacuum fluctuations have observable consequences, like the
Casimir force between mirrors in vacuum. This force is now measured with good
accuracy and agreement with theory when the effect of imperfect reflection of
mirrors is properly taken into account. We discuss the simple case of bulk
metallic mirrors described by a plasma model and show that simple scaling laws
are obtained at the limits of long and short distances. The crossover between
the short and long-distance laws is quite similar to the crossover between Van
der Waals and Casimir-Polder forces for two atoms in vacuum. The result
obtained at short distances can be understood as the London interaction between
plasmon excitations at the surface of each bulk mirror.
