question_answer

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

A)  15 cm                                  

B)  -15 cm

C)  - 30 cm                                

D)  30 cm

Correct Answer: C

Solution :

                 The focal length (f) of the combination of concave and convex lens is                 \[\frac{1}{f}=\frac{1}{{{f}_{1}}}+\frac{1}{{{f}_{2}}}\]                 where   \[{{f}_{1}}=+60\,cm\],                 \[{{f}_{2}}=-20\,\,cm\] \[\therefore \]  \[\frac{1}{f}=\frac{1}{60}-\frac{1}{20}\] \[\Rightarrow \]               \[\frac{1}{f}=\frac{-2}{60}\] \[\Rightarrow \]               \[f=-30\,\,cm\]