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The number of cardinal points in a thick lens is:
1. zero
2. two
3. three
4. six

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Correct Answer - Option 2 : two


  • Cardinal Point: The cardinal points consist of three pairs of points located on the optical axis of a rotationally symmetric, focal, optical system.
    • These are the focal points, the principal points, and the nodal points. 
    • For ideal systems, the basic imaging properties such as image size, location, and orientation are completely determined by the locations of the cardinal points; in fact, only four points are necessary: the focal points and either the principal or nodal points.
    • The only ideal system that has been achieved in practice is the plane mirror, however, the cardinal points are widely used to approximate the behaviour of real optical systems.
    • Cardinal points provide a way to analytically simplify a system with many components, allowing the imaging characteristics of the system to be approximately determined with simple calculations.


  • As in the thick lense, there are two cardinal points:
    • Nodal point 
    • Center point
  • However, the thin lens consists of 6 cardinal points respectively.


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