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

By the displacement method, an object and screen are fixed 100 cm apart. A convex lens forms a sharp image for two lens positions 40 cm apart. The focal length of the lens is:

Answer: 21 cm.

A 25 cm
B 35 cm
C 21 cm
D 24 cm

Correct answer: C. 21 cm

Explanation: f = (D2 − d2)/(4D) = (1002 − 402)/(4 × 100) = 8400/400 = 21 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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By the displacement method, an object and screen are fixed 100 cm apart. A convex lens forms a sharp image for two lens positions 40 cm apart. The focal length of the lens is:

Concept context

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