Question

# An eyeglass prescription calls for a lens with an optical power of $+2.9$ diopters. What is the focal length of this lens?

Solution

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The refractive power is

$R = 2.9 \ \mathrm{diopters}$

The focal length is,

\begin{align*} f &= \dfrac{1}{R} \\ &= \dfrac{1}{2.9 \ \mathrm{diopters}} \\ &= 0.3448 \ \mathrm{m} \\ &= \boxed{34.48 \ \mathrm{cm}} \end{align*}