Question

An object placed at a distance of 9cm from first principal focus of convex lens, produces a real image at a distance of 25cm from its second principal focus. Then focal length of lens is????

Answer 1

Here you go. Hope this will help.

Answer 2

Given:

Distance of the object = 9cm

Distance of the image = 25cm

To Find:

Focal length of the lens

Solution:

Let the focal length be = f

u = -f + a

v = 25 + f  

Lens formula = 1/f = 1/v - 1/u

Therefore,  

1/f = 1/ 25 + f - 1/ -f +9  

= f + 9 + f + 25 / ( f + 9 ) ( f + 25 )

= 2 ( f + 17)f = ( f + 9 ) ( f + 25 )

f² + 25f + 9f + 225 = 2f + 34f

f² = 225

f = 15cm

Answer: The focal length of the lens is 15cm.