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
2f
C
34f
D
43f
Medium
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Solution
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Correct option is D)
Water in between the lens can be considered to be a concave lens of refractive index of water as shown in the figure. This suggests that the radius of curvature of all the lenses is same.
Consider the convex lens-
f1=R(μg−1)2=R(23−1)2
hence R=f
For the concave lens,
Power=f11=(μwater−1)(R−2)=3f−2
Power of the lenses get added,
fcombination1=f1+3f−2+f1
Hence fcombination=43f
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