  The shock wave instability induced when interacting with a small waviness on
an interface was investigated analytically and numerically. The perturbation to
the shock was phenomenologically treated assuming this as the consequence of
the shock refraction. The instability develops in the form of wave-like
stretchings into the lower density medium followed with the loss of stability
in the flow behind it, and eventually evolving into an intense vortex
structure. The instability mode is aperiodical and unconditional, and either a
transition to another stable state or continuous development as a secondary
flow is possible. Among other interesting features are: a similarity law in the
spatial and temporal evolution of the perturbations with respect to the
interface curvature; the instability locus independence of the gas density
distribution thus identifying the interface conditions as the sole triggering
factor; the role of the density gradient in the instability evolution
discriminating between qualitatively different outcomes; and the possibility of
decay via non-viscous dumping mechanisms. The phenomenological connection
between the shock and the interface stability is discussed.
