Negative refraction, also known as Veselago lensing, was first predicted by Victor Veselago in 1968 (Veselago (1968)). Its unique effect has a great potential for both scientific and technological applications such as superlenses. Unlike the conventional positive refractive index, focusing effect can be observed by negative refraction. In this thesis, the focusing effect was investigated theoretically through on n-p junction in graphene. The opposite chirality of electrons and holes enable the negative refraction where electrons( holes) have their momentum parallel(anti-parallel) to the group velocity. The case when potential barrier is directed perpendicular to KK direction, where K and K are the Dirac points were considered. The Green’s functions were calculated analytically and derived the susceptibility using the Green’s functions for various positions of the sources and the receiver at various Fermi energies. The spatial Green’s functions were calculated analytically and derived the static susceptibility (response function).