Lens-Maker Formula: An amateur astronomer grinds a double convex lens whose surfaces have radii of curvature of magnitudes 40 cm and 60 cm. The glass has an index of refraction of 1.54. What is the focal length of this lens when used in the night air?
A. 43 cm
B. 130 cm
C. 89 cm
D. 44 cm
E. 220 cm
Answer: D
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