To apply the method of images, if we place an image charge -Q as shown in the figure 2.26, the system of two charges (essentially a dipole) will produce zero potential at the location of the conducting plane in the original problem. Solution of field and potential in the region plane (conducting plane in the original problem) will remain the same even the solution is obtained by solving the problem with the image charge as the charge and the boundary condition remains unchanged in the region .

Fig 2.26