Four lenses are made of glass and the radius of curvature of each face is given below. Which will have the greatest focal power

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20 cm convex 30 cm concave

15 cm convex and plane

10 cm convex and 15 cm convex

5 cm convex and 10 cm concave

answer is B.

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Detailed Solution

$\frac{1}{f} = (μ - 1) (\frac{1}{R_{1}} - \frac{1}{R_{2}})$

$P \propto (\frac{1}{R_{1}} - \frac{1}{R_{2}})$

$(1) \frac{1}{10} - \frac{1}{- 15} = \frac{5}{30} = \frac{1}{6} = 0.16$

$(2) \frac{1}{5} - \frac{1}{- 10} = \frac{3}{10} = 0.30$

$(3) \frac{1}{15} - \frac{1}{\infty} = \frac{1}{15} = . 06$

$(4) \frac{1}{20} - \frac{1}{- 30} = \frac{5}{60} = \frac{1}{12} = 0.08$

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