Correct Answer - Option 3 :
\(\dfrac{f_1 f_2}{f_1 + f_2}\)
CONCEPT:
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The focal length of the combination: When two lenses of focal length f1 and f2 are placed in contact, the effective focal length f of the combination is given by:
\( {1 \over f}= {1 \over f_1} +{1 \over f_2}\)
CALCULATION:
Given that Two thin lenses are of focal lengths f1 and f2
The focal length of the combination \( {1 \over f}= {1 \over f_1} +{1 \over f_2}\)
\(f=\dfrac{f_1 f_2}{f_1 + f_2}\)
So the correct answer is option 3.
- The focal length of the convex lens is positive and that of the concave lens is negative.