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What is the power of combination of convex lens and concave lens of equal focal length 25 cm?

Asked in GUJCET 2025 · Combinations and silvered lenses

• (1) Zero
• (2) 25 D
• (3) Infinite
• (4) 8 D

Step-by-step solution

Why the other options are wrong

• (2) comes from quotes the focal length's number as though it were a power. P=1/f with f in metres gives 1/(0.25)=4 D for each lens, not 25.
• (3) comes from this is the equivalent focal length, not the power. F=(f₁f₂)/(f₁+f₂) has f₁+f₂=0 here, so F really is infinite — and that is exactly why the power P=1/F is zero.
• (4) comes from adds the two magnitudes, 4+4, forgetting that the concave lens carries P=-4 D. The two cancel instead of reinforcing.

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