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

Two thin convex lenses of focal lengths 10 cm and 20 cm are placed coaxially 5 cm apart. The focal length of the combination is:

Answer: 8 cm.

A 8 cm
B 6.7 cm
C 12.5 cm
D 30 cm

Correct answer: A. 8 cm

Explanation: P = P1 + P2 − d P1 P2 = 10 + 5 − 0.05(10)(5) = 12.5 D (d in metres). f = 1/12.5 = 0.08 m = 8 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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Two thin convex lenses of focal lengths 10 cm and 20 cm are placed coaxially 5 cm apart. The focal length of the combination is:

Concept context

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