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The refractive index of a converging lens is 1.4. What will be the focal length of this lens if it is placed in a medium of same refractive index? Assume the radii of curvature of the faces of lens are R₁ and R₂ respectively.

Asked in JEE Main 16th March 2nd Shift 2021 · Lens maker's formula and power

• (1) zero
• (2) (R₁R₂)/(R₁-R₂)
• (3) 1
• (4) Infinite

Step-by-step solution

Why the other options are wrong

• (1) Confuses zero power with zero focal length.
• (2) Takes the relative index as 2 instead of 1, giving the air formula for index 2.
• (3) Gives the relative index 1, not a focal length.

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