question_answer

Two thin lenses of focal lengths \[{{f}_{1}}\] and \[{{f}_{2}}\] are in contact and coaxial. The power of the combination is                          [AIPMPT (S) 2008]

A)  \[\sqrt{\frac{{{f}_{1}}}{{{f}_{2}}}}\] 

B)                   \[\sqrt{\frac{{{f}_{2}}}{{{f}_{1}}}}\] 

C)  \[\frac{{{f}_{1}}+{{f}_{2}}}{{{f}_{1}}{{f}_{2}}}\]  

D)  \[\frac{{{f}_{1}}+{{f}_{2}}}{{{f}_{1}}{{f}_{2}}}\]

Correct Answer: C

Solution :

If two thin lenses of focal lengths  are placed in contact coaxially, then equivalent focal length of combination is

Power for the combination is