To have a simple microscope of linear magnification \( = 7.25 \) for the image to be formed at the near point \( D = 25\ \mathrm{cm} \), the focal length of its convex lens should be?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \( 4\ \mathrm{cm} \)
For a simple microscope with the image at the near point, the magnification is \( m = 1 + D/f \).
\( \displaystyle 7.25 = 1 + \frac{25\ \mathrm{cm}}{f} \)
\( \displaystyle \frac{25\ \mathrm{cm}}{f} = 6.25 \Rightarrow f = \frac{25}{6.25}\ \mathrm{cm} = 4\ \mathrm{cm} \)