  The theory of gravitational lensing has revealed many generic and fundamental
properties of compact objects like black holes and wormholes. In this article,
we utilize a recent formulation to compute the quantum effects on the
deflection angle of a light ray, namely, the Gauss-Bonnet theorem (GBT) to
explore the semiclassical gravitational effects in the spacetime of a
point-like global monopole and a cosmic string. Previously, the Gauss-Bonnet
theorem [Class. Quant. Grav. 25, 235009 (2008)] was proposed as an alternative
way to compute the deflection angle of light in a static, spherically symmetric
and asymptotically flat spacetime. In the present article we have used the
celebrated GBT that applied to the optical metric as well as the geodesic
method in computing the deflection angle. Interestingly one can observe that we
have found an exact result between GBT and the standard approach up to the
third-order contributions terms by modifying the domain of integration for
cosmic string and global monopole deflection angles. Finally we have considered
the time delay in the cosmic string/global monopole spacetime and found that
the delay in time is proportional to the linear mass density of the cosmic
string and global monopole parameter, respectively.
