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Geometrical Optics question

2023 · Q158
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Geometrical Optics question

2023 · Q158

NEETPhysicsGeometrical OpticsMCQ+4 / −1

In the figure shown here, what is the equivalent focal length of the combination of lenses (Assume that all layers are thin) ?

NEET 2023 Physics - Geometrical Optics Question 11 English

  1. A
    −40 cm-40 \mathrm{~cm}−40 cm
  2. B
    −100 cm-100 \mathrm{~cm}−100 cm
  3. C
    −50 cm-50 \mathrm{~cm}−50 cm
  4. D
    40 cm40 \mathrm{~cm}40 cm
View written solutionFree

Correct answer: B

NEET 2023 Physics - Geometrical Optics Question 11 English Explanation

Use $$\frac{1}{\mathrm{f}}=[\mu-1]\left[\frac{1}{\mathrm{R}_{1}}-\frac{1}{\mathrm{R}_{2}}\right]$$

$$\frac{1}{\mathrm{f}_{1}}=[1.6-1]\left[\frac{1}{\infty}-\frac{1}{20}\right]=\frac{-3}{100}$$

$$\frac{1}{\mathrm{f}_{2}}=[1.5-1]\left[\frac{1}{20}+\frac{1}{20}\right]=\frac{1}{20}$$

$$\frac{1}{\mathrm{f}_{3}}=\frac{-3}{100}$$

$$\frac{1}{f_{e q}}=\frac{1}{f_{1}}+\frac{1}{f_{2}}+\frac{1}{f_{3}}$$

$$\frac{1}{f_{e q}}=-\frac{3}{100}+\frac{1}{20}-\frac{3}{100}=\frac{-1}{100}$$

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