Question

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A convex lens form Real and Inverted image of triple magnification. If focal length of lens is 40cm then find object and image distance.
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Answer 1

Explanation:

Image formed is real and inverted

∴ Magnifications (m) = -1

Since the image is real, image distance is a positive value.

Image distance (v) = 50 cm

Let the object distance be u

Magnification in a lens (m) = v/u

-1 = 50/u

u = -50 cm

The needle is placed 50 cm in front of the convex lens.

Need to find the focal length (f)

According to the lens formula,

1/f = 1/v – 1/u

1/f = 2/50

1/f = 1/25

f = 25cm

Power of the lens p = 1/f

P = 1/25cm-1

P = 1/25 × 10-2 m-1

P = 4m-1

Since 1m-1 = 1D

∴ P = 4D