Question

Magnification of a convex lens 2.5 cm,if object kept at 9cm from pole of lens what is its focal length?​

Answer 1

Answer:

According to formula f = ( m/m-1) ×u

putting the values we get f = -15 cm

thanx

Answer 2

Given:

Magnification of a convex lens 2.5 cm, object kept at 9cm from pole of lens.

To find:

Focal length of lens

Calculation:

Magnification = 2.5

$= > \dfrac{v}{u} = 2.5 = \dfrac{5}{2}$

$= > v = \dfrac{5u}{2} \: \: \: \: ......(1)$

Applying Lens Formula:

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

$= > \: \dfrac{1}{f} = \dfrac{1}{ (\frac{5u}{2} )} - \dfrac{1}{u}$

$= > \: \dfrac{1}{f} = \dfrac{2}{ 5u} - \dfrac{1}{u}$

$= > \: \dfrac{1}{f} = \dfrac{2 - 5}{ 5u}$

$= > \: \dfrac{1}{f} = \dfrac{ - 3}{ 5u}$

$= > f = - \dfrac{5u}{3}$

Putting value of u:

$= > f = - \dfrac{5( - 9)}{3}$

$= > f = \dfrac{45}{3}$

$= > f = 15 \: cm$

So , final answer is:

$\boxed{ \rm{focal \: length \: of \: lens = 15 \: cm}}$