Question

23. The power of a combination of two lenses X and Y is 5D. If the focal length of lens X is 15 cm, state
the nature and focal length of lens Y.
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Answer 1

Explanation:

let the focal of X be f1 and Y be f2

$power \: = \frac{100}{f(cm)}$

$f \: = \frac{100}{p}$

$f \: = \frac{100}{5}$

$f \: = 20 \: cm$

for combinations of lens

$\frac{1}{f} = \frac{1}{f1} + \frac{1}{f2}$

$\frac{1}{20} = \frac{1}{15} + \frac{1}{f2}$

$\frac{1}{f2} = \frac{1}{20} - \frac{1}{15}$

$f2 = - 60 \: cm$