Question

an erect image 2.0cm high is formed 12 cm from a lens, the object being 0.5cm high. Find the focal length of the lens.

Answer 1

this the required answer f=-4cm.

Answer 2

Answer:

Focal length of the lens is 4 cm.

Explanation:

It is given that,

Height of image, h' = 2 cm

Image distance, v = -12 cm

Height of object, h = 0.5 cm  

We have to find the focal length of the lens. It is a convex lens because the image is large as compared to object. Firstly calculating magnification of lens as :

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

$u=\dfrac{-12\times 0.5}{2}$

u = -3 cm

Now using Mirror's formula as :

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

$\dfrac{1}{-12}-\dfrac{1}{-3}=\dfrac{1}{f}$

f = +4 cm

So, the focal length of this lens is 4 cm. Hence, this is the required solution.