Question

a screen is placed 90cm from as object. the image of the object on the screen is formed by a convex lens as two different position separated by 20cm.calculate the focal length of lens.
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Answer 1

Answer:

21.4 cm

Explanation:

The image of the object can be located on the screen for two-position of convex lens such that u and v are exchanged.

The separation between two positions of the lens is, d= 20 cm

u1 + v1 =90cm

v1 − u1 =20cm

Solving,

v1 =55cm, u1 =35cm

From lens formula,

1 _ 1 = 1

v u f

1 _ 1 = 1

55 -35 f

⇒ 1 = 35*55

f 90

⇒f=21.4cm