Question

A convex lens of focal length 20cm produces a real image twice the size of the the object. The image distance lens is?

Answer 1

it will be 40 because it is saying twice so just multiply with 2

Answer 2

Given:

Focal length ($f$) = $20$ cm

A real image produced is twice the object's size, hence $h_{1} = 2h_{0}$  

$\frac{h_{1} }{h_{0} } = m$ where $m$ is called the magnification.

To find:

Distance of the object from the lens

Formula used:

Lens formula $\frac{1}{f} = \frac{1}{v} - \frac{1}{u}$

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

Step-by-step explanation:

Since, $m = \frac{v}{u}$

$\frac{v}{u} = 2$

$v = 2u$

$\frac{1}{f} = \frac{1}{2u} - \frac{1}{u}$

$\frac{1}{20} = \frac{1+2}{2u}$

$\frac{1}{20} = \frac{3}{2u}$

$2u = 60$

$u = 30 cm$

Answer:

Therefore, the distance of the object from the lens is $30cm$