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What is the criteria to know whether a lens is thin or thick ? Suppose we have a lens of r1= 10 mm , r2= -10 mm and thickness is 5 mm. So, with this information what can we say about the lens whether it is thin or thick ?

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    $\begingroup$ Wiki: "For a thin lens, [the thickness] is much smaller than one of the radii of curvature". 5 is not probably much smaller than 10. $\endgroup$ – Jasper Feb 28 at 8:25
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    $\begingroup$ @Jasper That looks like it should be an answer. $\endgroup$ – David Z Feb 28 at 8:27
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According to Wikipedia

For a thin lens, [the thickness] is much smaller than one of the radii of curvature.

Here you have thickness 5 mm and radii of 10 mm. From my point of view, 50% is not "much smaller". I'd say that for something to be much smaller or larger than something else, it should at least be one order of magnitude smaller/larger, so a factor of ~10 would be needed.

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Thin lens is abstract construction it is just a plane with optical power. Another way to imagine it is lens with curvatures but inside the lens the ray height does not change.

Thick lens is a real lens with physical thickness and curvature.

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