Two identical glass (μ_g=3/2) equiconvex lenses of focal length f each are kept in contact. The space between the two lenses is filled with water (μ_w=4/3). The focal length of the combination is

1. $f$

2. $4f/3$

3. $3f/4$

4. $f/3$

Doubtly Asked 30 Oct 2026 NEET UG 2016 Phase 2 Practice test neet Save WhatsApp Report

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Answer: 3. $3f/4$

First find the radius of each lens, then the water lens, then combine powers.

Glass lens: $\dfrac1f=(\tfrac32-1)\left(\dfrac2R\right)=\dfrac1R$, so $R=f$.
Water between the two lenses forms a concave-faced (diverging) lens with both radii $R$: $\dfrac1{f_w}=(\tfrac43-1)\left(-\dfrac2R\right)=-\dfrac{2}{3f}$.

$$\frac1F=\frac1f+\frac1f-\frac{2}{3f}=\frac{4}{3f}\Rightarrow F=\frac{3f}{4}$$

Option (3).

Doubtly AI · 30 Oct 2026 AI-generated · not yet verified Report

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