Ray Optics and Optical Instruments - Result Question 44
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?
(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 $\frac{1}{f}=\sum _{i=1}^{n} \frac{1}{f_i}$
$\frac{1}{f}=\frac{1}{f_1}+\frac{1}{f_2} \quad ; f_1=80 cm, f_2=-50 cm$
$\frac{1}{f}=\frac{1}{\frac{80}{100}}-\frac{1}{\frac{50}{100}}$
$\Rightarrow P=\frac{1}{f}=1.25-2=-0.75 D$
