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Nodal Points

Definition: points of an optical system for which an incoming ray, directed at a nodal point, leaves the system with the same direction.

German: Knotenpunkte

Category: general optics

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nodal points of a thick lens
Figure 1: The nodal points of a thick lens.

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.

In the frequently encountered situation where the refractive index is the same in front of and behind the optical system, the nodal points coincide with the principal points.

Some examples for nodal points:

Seealso: cardinal points, Gaussian optics

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