A screen is placed 90 cm from an object. The image of an object on the screen is formed by a convex lens at two different locations separated by 20 cm . The focal length of the lens is

Question

A screen is placed 90 cm from an object. The image of an object on the screen is formed by a convex lens at two different locations separated by 20 cm . The focal length of the lens is, 18 cm, 21 .4 cm, 60 cm, 85 .6 cm

MARKS App · Accepted Answer

From displacement method D = 30 cm and d = 20 cm f = D 2 - d 2 4 D ⇒ f = 90 2 - 20 2 4 × 9 0 f = 110 × 70 4 × 90 ⇒ f = 2 1 . 4 cm

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