Question

A converging lens is placed in between an object and a screen. The magnification produced due to the lens is 2. If the distance between the object and the screen is 45 cm, then calculate its focal length (in cm).

Answer 1

Answer:

The distance between the object and mirror =10 cm and magnification is  

4

1

​  

 

Magnification is the ratio of the height of the image with the height of the object

=  

u

v

​  

 

u−v=10cm

v=u−10cm

⇒  

u

v

​  

=  

u

u−10

​  

=  

4

1

​  

 

⇒4u−40=u

u=  

3

40

​  

cm

v=  

3

10

​  

cm

Now,

By Lens formula:

f

1

​  

=  

v

1

​  

−  

u

1

​  

 

f

1

​  

=  

10

3

​  

−  

40

3

​  

 

f

1

​  

=  

40

9

​  

 

f=  

9

40

​  

cm

Focal length, f=4.4cm