  Gravitational lensing is a good probe into the topological structure of dark
gravitating celestial objects. In this paper, we investigate the light curve of
a light ray that passes through the throat of an Ellis wormhole, the simplest
example of traversable wormholes. The method developed here is also applicable
to other traversable wormholes. To study whether the light curve of a light ray
that passes through a wormhole throat is distinguishable from that which does
not, we also calculate light curves without the passage of a throat for an
Ellis wormhole, a Schwarzschild black hole, and an ultrastatic wormhole with
the spatial geometry identical to that of the Schwarzschild black hole in the
following two cases: (i) "microlensing," where the source, lens, and observer
are almost aligned in this order and the light ray starts at the source,
refracts in the weak gravitational field of the lens with a small deflection
angle, and reaches the observer, and (ii) "retrolensing," where the source,
observer, and lens are almost aligned in this order, and the light ray starts
at the source, refracts in the vicinity of the light sphere of the lens with a
deflection angle very close to $\pi$, and reaches the observer. We find that
the light curve of the light ray that passes through the throat of the Ellis
wormhole is clearly distinguishable from that by the microlensing but not from
that by the retrolensing. This is because the light curve of a light ray that
passes by a light sphere of a lens with a large deflection angle has common
characters, irrespective of the details of the lens object. This implies that
the light curves of the light rays that pass through the throat of more general
traversable wormholes are qualitatively the same as that of the Ellis wormhole.
