In Gaussian optics, one can define various types of cardinal points, including the nodal points. By definition, an input ray directed at a nodal point leads to an output ray which has the same direction, only possibly with a parallel offset. For that, an incoming beam from the input side must be directed to the front nodal point, and the corresponding output ray then appears to come from the back nodal point.
Some examples for nodal points:
- The nodal points of a curved interface between two optical media are located at the center of curvature of the surface – possibly far away from the surface.
- For a thin lens, the two nodal points coincide in the center of the length. Therefore, a ray directed to that center will go through the lens without any deflection or parallel offset.
Seealso: cardinal points, Gaussian optics