Chapter 27 - Optical Instruments - Problems and Conceptual Exercises - Page 975: 108

$C. 4.35 diopter$

We know that $\frac{1}{f}=\frac{1}{d_{\circ}}+\frac{1}{d_i}$ We plug in the known values to obtain: $\frac{1}{f}=\frac{1}{23cm}+\frac{1}{-\infty}$ $\implies \frac{1}{f}=\frac{1}{23cm}=0.23m$ Now $P=\frac{1}{f }$ $\implies P=\frac{1}{0.23m}$ $\implies P=4.35 diopter$