Question

It is required to throw an image, magnified two times, on a screen placed at distance of 6 cm from the object. Find the focal length and nature of the lens
required
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Answer 1

Answer

m=2

m=v/u=2

v=2u

also

v=u+6

2u=u+6

u=6

v=2u

v=12

the object distance is always negative

so

u=-6

also the image is on the same side of the object .

so

v=-12

using lens formula

1/f=1/v-1/u

1/f=1/-12-1/-6

1/f=1/12

f=12

Focal length is positive.

So,it is a convex lens.

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Answer 2

Explanation:

m=2

m=v/u=2

v=2u

also

v=u+6

2u=u+6

u=6

v=2u

v=12

the object distance is always negative

so

u=-6

also the image is on the same side of the object .

so

v=-12

using lens formula

1/f=1/v-1/u

1/f=1/-12-1/-6

1/f=1/12

f=12

Focal length is positive.

So,it is a convex lens.