Question

A lens of focal length 12 cm forms an errect images, three times the size of the object .the distance between the object and image.

Answer 1

Answer: Given m = 3 = -v/u

So v = 3u

by lens formula for convex lens

1/v = 1/u - 1/f

1 /3u = 1/ u - 1/12

1/u - 1 /3u = 1/12

2/3u = 1/12

u = 2 × 12/3

u = 8 cm

v = 3u = 24cm.

ANS is v - u = 24 - 8 = 16 cm.

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Answer 2

The distance between the object and the image is 16 cm.

Given,

Focal length = 12 cm.

The size of the image is 3 times the size of the object.

To Find,

The distance between the object and the image.

Solution,

We know that, Magnification (m) = -v/u = 3

⇒ v = -3u

Also, we know that, 1/f = 1/v - 1/u

⇒ 1/12 = 1/-3u - 1/(-u)

⇒ 1/12 = -1/3u + 1/u

⇒ 1/12 = 2/3u

⇒ u = 8 cm

∴ v = -3u = -3x8 = -24 cm

Distance between the object and the image = v + u = -24 + 8 = -16 cm.

Negative sign shows the direction of the image.

So, the distance between the object and the image = 16 cm.

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