Question

two thin convex lenses of focal length 15 cm and 30 cm are placed in contact what will be the power of combination

Answer 1

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$p = \frac{1}{f} \\ \\ = > \frac{1}{15} \: \: d \\ \\ and \: \frac{1}{30} \: \: d \\ \\ now... \\ \\ combination : \\ \frac{1}{30} + \frac{1}{15} = > \frac{3}{30} = > \frac{1}{10} \: \: d \\ \\ = > 0.1 \: \: d$


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Answer 2

p=100/f
power of combination=100/30+100+15=(100+200)/30=300/30=10
power of combination is +10 D