The least distance of distinct vision of a person is 50 cm. He wants to read a book placed at 25 cm. The focal length of the spectacles would be:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → +50 cm
The spectacle lens must form a virtual image of the book at the person’s near point (50 cm), so that the eye can see it clearly. Use the lens formula with the sign convention that distances are measured from the lens, positive along the direction of the incident light. The book is the object, so \( u = -25\,\mathrm{cm} \), and the virtual image is on the same side, so \( v = -50\,\mathrm{cm} \).
\( \displaystyle \frac{1}{f} = \frac{1}{v} - \frac{1}{u} = \frac{1}{-50} - \frac{1}{-25} \)
\( \displaystyle \frac{1}{f} = -\frac{1}{50} + \frac{2}{50} = \frac{1}{50}\,\mathrm{cm^{-1}} \)
\( f = +50\,\mathrm{cm} \)
A positive focal length means a convex lens.
Note: A similar question was also asked in CUET (UG) 2023 on 30-05-2023 (Slot 2).
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic was removed from the latest NCERT book (2026-27), and it also appears to be out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
The Eye – …
