1.

Two thin lenses of focal lengths f1 and f2 are in contact.  The focal length of this combination is   [MP PET 2002]

A.            \[\frac{{{f}_{1}}{{f}_{2}}}{{{f}_{1}}-{{f}_{2}}}\]          
B.            \[\frac{{{f}_{1}}{{f}_{2}}}{{{f}_{1}}+{{f}_{2}}}\]
C.            \[\frac{2{{f}_{1}}{{f}_{2}}}{{{f}_{1}}-{{f}_{2}}}\]       
D.            \[\frac{2{{f}_{1}}{{f}_{2}}}{{{f}_{1}}+{{f}_{2}}}\]
Answer» C.            \[\frac{2{{f}_{1}}{{f}_{2}}}{{{f}_{1}}-{{f}_{2}}}\]       


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