Question

An object 30 m tall is placed on the principal axis of a convex lens its 10cm tall inverted image is formed on the screen placed at a distance of 15cm from the lens calculate the focal length of the lens

Answer 1

Magnication of lens = v / u

But since the image formed is inverted. m = I / O = v / u

m = 10 / 30 = v / u

- 1 / 3 = v / u [ Image is inverted ]

- 1 / 3 = 15 / u

u = -45 cm

1/ f = 1 / v - 1 / u

1 / f = 1 / 15 - 1 / (-45)

1 / f = 3 / 45 + 1 / 45

1 / f = 4 / 45

f = 45 / 4

f = 11.25 cm

Ans: 11.25 cm

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Answer 2

Explanation:

height of image = -10

height of object = 30

image distance = 15