  For dark compact objects such as black holes or wormholes, the shadow size
has long been thought to be determined by the unstable photon sphere (region).
However, by considering the asymmetric thin-shell wormhole (ATSW) model, we
find that the impact parameter of the null geodesics is discontinuous through
the wormhole in general and hence we identify novel shadows whose sizes are
dependent of the photon sphere in the other side of the spacetime. The novel
shadows appear in three cases: (A2) The observer's spacetime contains a photon
sphere and the mass parameter is smaller than that of the opposite side; (B1,
B2) there' s no photon sphere no matter which mass parameter is bigger. In
particular, comparing with the black hole, the wormhole shadow size is always
smaller and their difference is significant in most cases, which provides a
potential way to observe wormholes directly through Event Horizon Telescope
with better detection capability in the future.
