Question

A converging lens has the focal length of 25 cm. A 10-cm object is placed at 30 cm in front of the lens. How far is the image from the lens? What is the size of the image?

Answer 1

Explanation:

Given that,

Focal length of a converging lens, f = +25 cm

Size of the object, h = 10 cm

Object distance, u = -30 cm

We need to find the image distance and the size of the image.

Using lens formula, $\dfrac{1}{v}-\dfrac{1}{u}=\dfrac{1}{f}$, v is image distance

$\dfrac{1}{v}=\dfrac{1}{f}+\dfrac{1}{u}\\\\\dfrac{1}{v}=\dfrac{1}{25}+\dfrac{1}{(-30)}\\\\v=150\ cm$

Magnification,

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

h' is the size of image

$h'=\dfrac{vh}{u}\\\\h'=\dfrac{150\times 10}{(-30)}\\\\h'=-50\ cm$

So, the image distance is 150 cm the size of image is 50 cm.