  We report on observations of an extreme-ultraviolet (EUV) wave event in the
Sun on 2011 January 13 by \emph{Solar Terrestrial Relations Observatory}
(\emph{STEREO}) and \emph{Solar Dynamics Observatory} (\emph{SDO}) in
quadrature. Both the trailing edge and the leading edge of the EUV wave front
in the north direction are reliably traced, revealing generally compatible
propagation velocities in both perspectives and a velocity ratio of about 1/3.
When the wave front encounters a coronal cavity near the northern polar coronal
hole, the trailing edge of the front stops while its leading edge just shows a
small gap and extends over the cavity, meanwhile getting significantly
decelerated but intensified. We propose that the trailing edge and the leading
edge of the northward propagating wave front correspond to a non-wave coronal
mass ejection (CME) component and a fast-mode magnetohydrodynamic (MHD) wave
component, respectively. The interaction of the fast-mode wave and the coronal
cavity may involve a mode conversion process, through which part of the
fast-mode wave is converted to a slow-mode wave that is trapped along the
magnetic field lines. This scenario can reasonably account for the unusual
behavior of the wave front over the coronal cavity.
