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

G1 Physics · Accepted Answer

Step 1: For one lens in air with equal radii R, 1/f = (3/2 − 1)(2/R) = 1/R, so f = R. Step 2: The combination has four refracting surfaces; add signed surface powers in the air–glass–water–glass–air sequence. Step 3: Resulting power = 4/(3R), giving F = 3R/4 = 3f/4. Final answer: (D) 3f/4.