Ray Optics and Optical Instruments - Result Question 36
36. Two thin lenses of focal lengths $f_1$ and $f_2$ are in contact and coaxial. The power of the combination is:
[2008]
(a) $\sqrt{\frac{f_1}{f_2}}$
(b) $\sqrt{\frac{f_2}{f_1}}$
(c) $\frac{f_1+f_2}{2}$
(d) $\frac{f_1+f_2}{f_1 f_2}$
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Answer:
Correct Answer: 36. (d)
Solution:
- (d) The focal length of the combination
$\frac{1}{f}=\frac{1}{f_1}+\frac{1}{f_2}$
$\therefore$ Power of the combinations,
$P=\frac{f_1+f_2}{f_1 f_2}(\because P=\frac{1}{f})$
