Question

Two thin lenses, one of focal length $+60cm$ and the other of focal length $-20cm$ are put in contact. The combined focal length is

• A. $+15cm$
• B. $-15cm$
• C. $+30cm$
• D. $-30cm$

Answer 1

The correct option is D $-30cm$

If combined focal length is $f$ then $\left(1/f\right)=\frac{1}{60}+\frac{1}{-20}=(-1/30)$ so $f=-30cm$

Option D is correct.