  Wave effects can be important for the gravitational lensing of gravitational
waves. In such a case, wave optics must be used in stead of geometric optics.
We consider a plane wave entering a lens object and solve numerically the wave
equation for three lens models: the uniform density sphere, the singular
isothermal sphere, and the Hernquist model. By comparing our numerical
solutions with the analytical solutions under the thin lens approximation, we
evaluate the error of this approximation. The results show that the relative
error of the thin lens approximation is small if the geometrical thickness of
the lens is much smaller than the distance between the lens and the observer.
