Lens in Air or in Vacuum

Let ci = 1/Ri, c2 = 1/R2 and d be the curvatures and axial thickness of a lens with refractive index n. From (1.20a) to (1.21) we obtain K1 — (n — 1)/R1 and K2 = —(n — 1)/R2, hence by substitution in the above equation, the power of a lens in air is

However, the refractive index of a vacuum is approximatively na = 1.0003 under normal conditions of pressure and temperature. But it is a universal practice in optical design to specify the refractive indices of glasses with respect to air, therefore no correction of its refractive index is required for the above power of a lens working in air.

Hence, if a lens is designed for working under vacuum, then, in (1.28a), the quantity (n + na - 1) must be substituted to (n - 1). This entails

for the power of a lens in vacuum.

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