An equiconvex lens has radius of curvature R = 20 cm and refractive index n = 3/2. Light enters from air. What is the focal length (in cm) of the lens?

Correct answer: 20

Solution

An equiconvex lens has both surfaces curved outward. By convention R1 = +R and R2 = -R. The Lensmaker's equation 1/f = (n-1)[1/R1 - 1/R2] gives 1/f = (n-1)(2/R). Substituting n = 3/2 and R = 20 cm yields f = 20 cm.

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