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Abbe’s Invariant

In paraxial optics each single refracting surface satisfies the Abbe’s Invariant Q in the paraxial Area, that relates the front focal distance s of an axial object point with the back focal distance s’ of it’s conjugated point behind the surface

Q = n (\frac{1}{r}-\frac{1}{s}) = n' (\frac{1}{r}-\frac{1}{s'})

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