question_answer

Two thin lenses of focal lengths \[{{f}_{1}}\] and \[{{f}_{2}}\] are in contact and coaxial. The combination is equivalent to a single lens of power [MP PET 1996, 98;  MP PMT 1998; DCE 2000; UP SEAT 2005]

A)            \[{{f}_{1}}+{{f}_{2}}\]       

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

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

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