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A symmetric thin biconvex lens is cut into four equal parts by two planes AB and CD as shown in figure. If the power of original lens is 4D then the power of a part of the divided lens is

Asked in JEE Main 22nd Jan 2nd Shift 2025 · Lens maker's formula and power

Figure: Lens maker's formula and power
Answer: (1) 2D

Step-by-step solution

Cut along AB (the principal axis) only reduces the aperture: power unchanged.

Cut along CD (plane of the lens) leaves each piece with one curved face and one flat face: a plano-convex part.

Original: P=(μ-1)2/R=4 D. Plano-convex part: P'=(μ-1)1/R=P/2.

P'=2 D

Why the other options are wrong

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