Gravitational lensing of Continuous Gravitational Waves

Continuous gravitational waves are analogous to monochromatic light and therefore could be used to detect wave effects like interference or diffraction. This would be possible with strongly lensed gravitational waves. This article reviews and summaries the theory of gravitational lensing in the context of gravitational waves in two different regimes: geometric optics and wave optics, for two widely used lens models such as point mass lens and Singular Isothermal Sphere (SIS). Observable effects due to wave nature of gravitational waves are discussed. As a consequence of interference GWs produce beat patterns which might be observable with the next generation detectors like ground based Einstein Telescope, Cosmic Explorer or space-borne LISA, DECIGO. This will provide us a opportunity to estimate the properties of lensing system and other cosmological parameters with alternative techniques. Diffractive microlensing could become a valuable method to search for intermediate mass black holes formed in the centers of globular clusters. We also point to an interesting idea of detecting the Poisson-Arago spot proposed in the literature.