An object is placed 12 cm in front of a convex mirror of focal length 18 cm, the value of the image distance is
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 7.2 cm
Use the mirror formula with the Cartesian sign convention (distances measured from the pole; those behind the mirror, in the direction of the incident light, are positive). For a convex mirror the focus is behind the mirror, so \( f \) is positive; the object is in front, so \( u \) is negative.
\( f = +18\,\mathrm{cm} \)
\( u = -12\,\mathrm{cm} \)
\( \displaystyle \frac{1}{v} + \frac{1}{u} = \frac{1}{f} \;\Rightarrow\; \frac{1}{v} = \frac{1}{18} + \frac{1}{12} \)
\( \displaystyle \frac{1}{v} = \frac{2 + 3}{36} = \frac{5}{36} \)
\( v = +7.2\,\mathrm{cm} \)
The image is 7.2 cm behind the mirror, virtual and erect, as always for a convex mirror.
Note: NTA picked this question from an NCERT (2026-27) exercise and changed the figures. Do not ignore NCERT while practising numericals.
NCERT (2026-27):
Exercises – …
