Lensmaker's equation: 1/f = (n-1)(1/R 1 - 1/R 2 ). For plano-convex lens (R 2 = infinity), if n=1.5 and R 1 =20 cm, focal length:

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Lensmaker's equation: 1/f = (n-1)(1/R1 - 1/R2). For plano-convex lens (R2 = infinity), if n=1.5 and R1=20 cm, focal length:

Answer: 40 cm.

A 10 cm
B 20 cm
C 30 cm
D 40 cm

Correct answer: D. 40 cm

Explanation: 1/f = (1.5-1)(1/20 - 0) = 0.5/20 = 1/40. f = 40 cm.

Concave mirror ray diagram with an object drawn as a candle on the principal axis, the focal point F marked, and the parallel ray and focal ray reflecting to form an inverted real image

Concave mirror ray construction: a ray parallel to the axis reflects through F, and a ray through F reflects parallel, locating the inverted real image. Image: Maxmath12, CC0, via Wikimedia Commons.

Concept context

Reflection, refraction, lenses, mirrors, total internal reflection.

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Lensmaker's equation: 1/f = (n-1)(1/R<sub>1</sub> - 1/R<sub>2</sub>). For plano-convex lens (R<sub>2</sub> = infinity), if n=1.5 and R<sub>1</sub>=20 cm, focal length:

Concept context

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