Two identical glass (μe=3/2) lenses of focal length f each keeps in contact. The space between the two lenses is filled with water (μw=4/3). The focal length of the combination is.
4f/3
f
f/3
3f/4
A
f
B
4f/3
C
3f/4
D
f/3
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Solution
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The water in between two convex lenses in contact acts like concave lens.
Focal length of convex lens is given by
1f=(μe−1)2R=1R
Focal length of concave lens is given by
1f′=−(μw−1)2R=−23f
⟹f′=−3f2
Equivalent focal length is given by
P=1feq=1f−23f+1f=43f
⟹feq=3f4
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