Discussion
Which of the following lenses would you prefer to read small letters in a dictionary ?
Answer: C
The right lens is a convex lens of focal length 5cm.
A convex lens used as a simple magnifier gives an enlarged, upright image. Its magnifying power is greater when the focal length is shorter, roughly \(1 + D/f\) with \(D = 25\) cm:
\[ \begin{aligned} f = 5\ \text{cm} &: \ 1 + 25/5 = 6 \\ f = 50\ \text{cm} &: \ 1 + 25/50 = 1.5 \end{aligned} \]
So the 5 cm convex lens magnifies far more than the 50 cm one. Concave lenses always form smaller images, so they cannot magnify small print at all.
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