Question

a convex lens of focal length 20cm produces a real image two times the size of an object then find the distance of the object from the lens.

Answer 1

M=-v/u

2=-v/u

2u=-v

V=-2u

By mirror formula,

I/v + 1/u =1/f

1/-2u + 1/u =1/20

-1/2u +1/u =1/20

-1+2/2u=1/28

1/2u =1/28

2u =28

u =-14cm

V=-28cm

I HOPE IT HELPS YOU!

Answer 2

The object is placed at a distance of 30 cm from the lens.

Explanation:

Given that,

Focal length of the convex lens, f = +20 cm

It produces a real image two times the size of an object, m = -2 (negative as it forms real image)

Magnification of a lens is given by :

$m=\dfrac{v}{u}$

$-2=\dfrac{v}{u}$

$v=-2u$.........(1)

The lens formula is given by :

$\dfrac{1}{f}=\dfrac{1}{v}-\dfrac{1}{u}$

$\dfrac{1}{(-2u)}=\dfrac{1}{20}+\dfrac{1}{u}$

u = -30 cm

So, the object is placed at a distance of 30 cm from the lens. Hence, this is the required solution.

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Lens formula

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