Question

A convex lens produces an image of an object on a screen with a magnification of 1/2. When the lens
is moved 30 cm away from the object, the magnification of the image is 2. The Focal length
of the lens is
(1) 20 cm
(2) 25 cm
(3) 30 cm
(4) 35 cm​

Answer 1

Answer:

1) 20 cm

Explanation:

We know that

m= f−v by f

As given,

1/2 = f-v by f

f=2(f−v)

f=2f−2v

2v=2f−f

2v=f

v=(f/2)

Now we havem =2

and v1=(v+30)

⇒m= f-v by f

Rightarrow 2= f−(v+30) by f

2=f-(f/2+30) by f from i

2f=f−f/2+30

⇒2f= f by 2+30

⇒2f− f by 2 =30cm

⇒4f−f by 2 =30cm

⇒ 3f by 2 =30

⇒3f=60

⇒f=20cm

I hope it helped you