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:

  1. (d) The focal length of the combination

$\frac{1}{f}=\frac{1}{f_1}+\frac{1}{f_2}$

$\therefore$ Power of the combinations,

$P=\frac{f_1+f_2}{f_1 f_2}(\because P=\frac{1}{f})$



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