Question

# Two identical glass (μg=3/2) equiconvex lenses of focal length f are kept in contact. The space between the two lenses is filled with water (μw=4/3). The focal length of the combination is

A
f
B
f2
C
4f3
D
3f4
Solution
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#### Let R be the radius of curvature of each surface. Then1F=(1.5−1)(1R+1R)For the water lens,1f′=(43−1)(−1R+1R)=13(−2f);1f′=−23fNow, using 1F=1f1+1f2+1f3, we have1F=1f+1f+1f′=2f−23f=43f⇒F=3f4.

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