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A thin convex lens made from crown glass (μ = 3/2) has focal length f. When it is measured in two different liquids having refractive indices 4/3 and 5/3, it has the focal lengths f₁ and f₂ respectively. The correct relation between the focal lengths is

Asked in JEE Main 2014 · Lens maker's formula and power

• (1) f₁ and f₂ both become negative
• (2) f₁ = f₂ < f
• (3) f₁ > f and f₂ becomes negative
• (4) f₂ > f and f₁ becomes negative

Step-by-step solution

Why the other options are wrong

• (1) Forgets that in the μ = 4/3 liquid the lens is still denser, so it stays converging.
• (2) Assumes the liquid changes nothing about the sign and shortens both focal lengths.
• (4) Swaps the two liquids: it is the denser liquid (5/3) that makes the lens diverging.

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