Discussion
Which of the following lenses would you prefer to read small letters in a dictionary ?
Answer: C
The best choice is a convex lens of focal length 5 cm.
A magnifying glass is a convex lens with the object placed within its focal length, giving an enlarged, erect, virtual image. Its magnification is about \(1 + \frac{D}{f}\), with \(D = 25\) cm, so a shorter focal length magnifies more:
\[ \begin{aligned} f = 5\ \text{cm} &: \ 1 + \frac{25}{5} = 6 \\ f = 50\ \text{cm} &: \ 1 + \frac{25}{50} = 1.5 \end{aligned} \]
Concave lenses always form smaller images, so neither concave option can magnify small print.
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