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

A convex lens of glass (n = 1.5) has focal length 20 cm in air. When fully immersed in water (n = 1.33), its focal length becomes about:

Answer: 78 cm.

A 78 cm
B 20 cm
C 40 cm
D 15 cm

Correct answer: A. 78 cm

Explanation: f is proportional to 1/(nrel − 1). nrel in water = 1.5/1.33 = 1.128, so fwater = 20 × (0.5/0.128) ≈ 78 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 convex lens of glass (n = 1.5) has focal length 20 cm in air. When fully immersed in water (n = 1.33), its focal length becomes about:

Concept context

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