Ray Optics And Optical Instruments Ques 43
43. 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?
[1996]
(a) $+6.5 $ $D$
(b) $-6.5 $ $D$
(c) $+7.5 $ $D$
(d) $-0.75 $ $D$
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Answer:
Correct Answer: 43.(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 $ $D$
BETA
