Ray Optics and Optical Instruments - Result Question 44

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46. A convex lens of focal length $80 cm$ and a concave lens of focal length $50 cm$ are combined together. What will be their resulting power?

======= ####46. A convex lens of focal length $80 cm$ and a concave lens of focal length $50 cm$ are combined together. What will be their resulting power?

3e0f7ab6f6a50373c3f2dbda6ca2533482a77bed:content/english/neet-pyq-chapterwise/physics/ray-optics-and-optical-instruments/ray-optics-and-optical-instruments—result-question-44.md (a) $+6.5 D$

(c) $+7.5 D$

(b) $-6.5 D$

(d) $-0.75 D$

[1996]

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Answer:

Correct Answer: 46. (d)

Solution:

(d) We know that $\frac{1}{f}=\sum _{i=1}^{n} \frac{1}{f_i}$

$\frac{1}{f}=\frac{1}{f_1}+\frac{1}{f_2} \quad ; f_1=80 cm, f_2=-50 cm$

$\frac{1}{f}=\frac{1}{\frac{80}{100}}-\frac{1}{\frac{50}{100}}$

$\Rightarrow P=\frac{1}{f}=1.25-2=-0.75 D$