JEE Main 2022 · 24 Jun · Shift 1 · Physics Q52

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PhysicsInteger+4 / -1Question 52 of 90

Two identical thin biconvex lenses of focal length 15 cm and refractive index 1.5 are in contact with each other. The space between the lenses is filled with a liquid of refractive index 1.25. The focal length of the combination is ______ cm.

Your answer

Answer and worked solution

Official answer: 10

Radius of each lens. $\dfrac1{15}=(1.5-1)\dfrac2R$ , so $R=15$ cm.

Liquid lens. The liquid fills a biconcave gap with both radii 15 cm: $\dfrac1{f_l}=(1.25-1)\left(-\dfrac2{15}\right)=-\dfrac1{30}$ .

Combination. $\dfrac1F=\dfrac2{15}-\dfrac1{30}=\dfrac1{10}$ , so $F=10$ cm.

Answer: 10

Topics:Combination of lenses and mirrors Thin lenses and lens maker's formula

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JEE Main 2022 · 24 Jun · Shift 1, Physics question 52