Question

Where should an object be placed in front of a convex lens to get a real image of the size of the object?<br />(a) At the principal focus of the lens<br />(b) At twice the focal length<br />(c) At infinity<br />(d) Between the optical centre of the lens and its principal focus.​

Answer 1

At twice focal length

Answer 2

Given that,

An object be placed in front of a convex lens.

We need to find how get the real image

We know that,

If an object is placed in front of convex lens at twice of the focal length then the image will be real. the size of image will be same of size of object.

Suppose, An object placed 40 cm in front of convex lens and focal length of lens is 20 cm.

We need to find the image distance

Using lens formula

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

Put the value into the formula

$\dfrac{1}{20}=\dfrac{1}{v}-\dfrac{1}{-40}$

$\dfrac{1}{v}=\dfrac{1}{20}-\dfrac{1}{40}$

$v=40\ cm$

The image is real and the size of image and object is equal.

Hence, The object will be placed at twice the focal length

(b) is correct option.