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

A plano-convex lens of glass (n=1.5) has curved surface R=20 cm. Placed with flat surface in water (n=1.33). Focal length:

Answer: 120 cm.

A 40 cm
B 80 cm
C 120 cm
D 160 cm

Correct answer: C. 120 cm

Explanation: Using lensmaker: 1/f = (n₂-n₁)/n₁ x (1/R₁-1/R₂). For curved side (R₁=20) in water (n₁=1.33): 1/f = (1.5-1.33)/1.33 x (1/20 - 0) = 0.17/1.33 x 1/20 = 0.0064. f = 156 cm approx 160 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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A plano-convex lens of glass (n=1.5) has curved surface R=20 cm. Placed with flat surface in water (n=1.33). Focal length:

Concept context

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