⚛️ Physics · Ray Optics and Optical Instruments · NEET & JEE

An equiconvex lens is made of glass (n = 1.5) with each surface of radius of curvature 20 cm. Its focal length is:

Answer: 20 cm.

A 40 cm
B 10 cm
C 20 cm
D 15 cm

Correct answer: C. 20 cm

Explanation: 1/f = (n − 1)(1/R<sub>1</sub> − 1/R<sub>2</sub>) = 0.5(1/20 − 1/(−20)) = 0.5 × (2/20) = 0.05, so f = 20 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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An equiconvex lens is made of glass (n = 1.5) with each surface of radius of curvature 20 cm. Its focal length is:

Concept context

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