If two thin convex lenses are kept in contact, the focal length of the combination is:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → smaller than either focal length
When two thin lenses with focal length $f_1$ and $f_2$ are kept in contact,
$\frac{1}{F_{net}}=\frac{1}{f_1}+\frac{1}{f_2}$
Since, the reciprocal of the combined focal length is greater than the reciprocal of either $f_1$ or $f_2$, the combined focal length is smaller than either $f_1$ or $f_2$.