A convex lens of focal length 30 cm is in contact with a concave lens of focal length 20 cm. Is the system working as a converging or diverging lens. What is the focal length of the combination.
Answer & explanation
Correct answer: option 2
Focal length of the combination is $\frac{1}{F} = \frac{1}{f_1}+\frac{1}{f_2} = \frac{1}{30} + \frac{1}{-20} = \frac{2 -3}{60} = \frac{1}{-60}$
$\Rightarrow F = -60cm$
Since focal length is negative hence the combination is diverging.
The correct answer is Option (2) → Diverging, 60 cm