A convex lens used as a simple microscope gives a magnification of 6 when the image is at the near point (25 cm). It is replaced by a lens of half its focal length, used in the same way. The magnification now is
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 11
With the image at the near point, a simple microscope gives \( m=1+D/f \). The first lens fixes \( f \). The new lens has half this focal length.
\( \begin{aligned} 6 &= 1+\frac{25}{f} \\ f &= 5\ \mathrm{cm} \end{aligned} \)
\( f^{\prime}=2.5\ \mathrm{cm} \)
\( \begin{aligned} m^{\prime} &= 1+\frac{D}{f^{\prime}} \\ &= 1+\frac{25}{2.5} \\ &= 11 \end{aligned} \)
Hence, the magnification with the new lens is 11.
NCERT (2026-27):
The microscope – “Now according to our sign convention, v is negative, and is equal in magnitude to D. Thus, the magnification is”
\( \displaystyle m=1+\frac{D}{f} \)\( \qquad (9.39) \)