For expanding the flexure z, the absolute value of the coefficient 5 must be larger than unity, i.e. |5| = t0/a\®\ > 1. From definition (9.26), the pseudo-power m is related to the power K by m = 2a(ci - c2) = aK/2(n- 1),

where n is the refractive index. This entails |5| = 2(n - 1)t0/a2|K|. Considering the f-number of the lens, Q = f/D = f/2a = 1/2a|K|, the condition for 5 writes

For instance, for a lens at f/10 with To = t0/a = 1/10 and n = 3/2, we obtain m -1/20 and the condition gives |5| = 2. If the lens is positive, then 5 = 2.

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