Two identical thin plano-convex glass lenses (refractive index 1.5)… | NEET 2015 (AIPMT, Cancelled Paper)

Q71NEET 2015 · AIPMT · CANCELLED PAPERMedium

Combination of Lenses

Two identical thin plano-convex glass lenses (refractive index 1.5) each having radius of curvature of 20 cm are placed with their convex surfaces in contact at the centre. The intervening space is filled with oil of refractive index 1.7. The focal length of the combination is

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Answer: (a)

Each plano-convex glass lens: $\frac{1}{f_g} = (1.5-1)\frac{1}{20} = \frac{1}{40}$ .
The oil between the convex faces forms a biconcave lens: $\frac{1}{f_o} = (1.7-1)\left(-\frac{1}{20} - \frac{1}{20}\right) = -\frac{7}{100}$ .
$\frac{1}{F} = \frac{2}{40} - \frac{7}{100} = -\frac{2}{100}$
So the answer is (a) $-50\text{ cm}$ .

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Combination of Lenses question from NEET 2015 (AIPMT, Cancelled Paper)

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