  For the observer at infinity, a Schwarzschild black hole serves as an
attractive opaque disk with a radius of 3$\sqrt{3} M$ that will produce the
diffraction pattern of gravitational waves (GWs). In this study, we demonstrate
that a bright spot, which is a diffraction effect analogous to the
Poisson-Arago spot in optics, will appear when an ingoing (quasi-) plane GW is
diffracted by a Schwarzschild black hole. Here, we propose the diffraction
effect of the GWs described by the exact diffraction solution of the GWs using
the Heun function. For the first time, the Fresnel half-wave zone method is
proposed to calculate the angular part of the GW scattering stripes for the
observer at infinity. The prospect of observing the diffraction bright spot is
discussed with an eikonal approximation. For normal incidence (quasi)-plane
waves with 100 Hz (0.1 Hz) frequency diffracted by the central black hole of
the Milky Way, the time delay between the Earth bathed in a bright spot and the
minimum of the first dark stripe is 3.86 (3860) days. We will witness the
second bright fringe (40% amplitude of the central bright spot) after 6.2
(6200) days. This new diffraction pattern involving the early phase of
inspirals and pulsars as continuous gravitational wave sources is a potential
scientific target for future space-and ground-based gravitational wave
detectors, respectively.
