Two thin lenses of same focal length (f), one convex and the other concave, are placed in contact. The equivalent focal length of the combination will be:
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Question

Two thin lenses of same focal length (f), one convex and the other concave, are placed in contact. The equivalent focal length of the combination will be:

G1 Physics · Accepted Answer

Step 1: Powers of thin lenses in contact add: 1/F = 1/f₁ + 1/f₂. Step 2: For equal magnitudes but opposite types, f₁ = +f, f₂ = −f. Step 3: 1/F = 1/f − 1/f = 0, so F → ∞. Final answer: (B) Infinite.