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Chapter 2:
Best Shape for a Thin Lens
2.1 Concept of Thin Lens The “thin lens” concept is an extremely useful one for quick preliminary layout calculations and initial performance analyses. “Thin” means that the thickness of the lens is ignored in the calculations. The power ϕ of a lens is the reciprocal of its focal length f. The reciprocal of a surface radius R is called the surface curvature c. With t being the lens thickness and n the index of refraction of the lens material, we can state 1 f =ϕ=(n−1)[c 1 −c 2 +(n−1) n c 1 c 2 t]. Neglecting the thickness t yields the power of the thin lens ϕ=(n−1)(c 1 −c 2 )=(n−1)c. In this equation, c is called the net curvature and is the difference between the front and rear surface curvatures, i.e., c=c 1 −c 2
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