The following figure shows different arrangements of two identical pieces of thin Plano-convex lenses. The refractive index of the liquid used is less than that of the glass. The effective focal lengths in the three cases are related as :
A
f1=f2>f3
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B
f1≠f2≠f3
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C
f1=f2<f3
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D
f1=f2=f3
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Solution
The correct option is Bf1=f2<f3 Let the focal length of each piece be, f
Then, the focal length of the combination will be,
1F=1f+1f
So, F=f2
Now, in the third case, the presence of water between the two surfaces, acts as a diverging mirror and thereby increases the focal length of the combination