nodal points
A ray of light traveling in the direction of the front nodal point will be refracted by the lens such that, when it emerges on the other side and refracted again, it will appear to be coming from the back nodal point.
A ray of light traveling in the direction of the front nodal point will be refracted by the lens such that, when it emerges on the other side and refracted again, it will appear to be coming from the back nodal point.