Question

An object has image thrice of its original size when kept at $8cm$ and $16cm$ from a convex lens. Focal length of the lens is

• A. 8 cm
• B. 16 cm
• C. Between 8 cm and 16 cm
• D. Less than 8 cm

Answer 1

Answer: (C)

$m=\pm 3$, using $m=\frac{f}{f+u}$
For virtual image $3=\frac{f}{f-8}$ $\dots$(i)
For real image $-3=\frac{f}{f-16}$ ....(ii)
Solving (i) and (ii) we get $f=12cm$