A) decreases
B) increases
C) becomes
D) remains the same
Correct Answer: A
Solution :
When two lenses are placed coaxially at a distance d from each other, then equivalent focal length, (F) is given by \[\frac{1}{F}=\frac{1}{{{f}_{1}}}+\frac{1}{{{f}_{2}}}-\frac{1}{{{f}_{1}}{{f}_{2}}}\] When the lenses are put together. \[d=0\] Hence, the focal length of the combination will decrease.
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