Question

a lens of focal length 20 cm forms an inverted image at a distance 60 cm from the lens
1. Identify the lens
2. How far is the lens present in front of object
3. Calculate the magnification of the image

Answer 1

The nature of lens is Convex , inverted , same   size of object

The distance of object from lens is - 30 cm

The magnification of image is - 2   .

Explanation:

Given as :

The focal length of lens = f = 20 cm

The image is inverted from the center of lens at distance = v = 60 cm

Let The distance of object from lens = u  cm

From lens formula

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

Or, $\dfrac{1}{20cm}$   =   $\dfrac{1}{60cm}$   -  $\dfrac{1}{u}$

Or, $\dfrac{1}{u}$   =  $\dfrac{1}{60cm}$  -  $\dfrac{1}{20cm}$

Or, $\dfrac{1}{u}$   =  $\dfrac{1-3}{60cm}$

Or, $\dfrac{1}{u}$  =  $\dfrac{-2}{60cm}$

Or, $\dfrac{1}{u}$  = $\dfrac{-1}{30cm}$

∴   u = - 30 cm

So, The distance of object from lens = u = - 30  cm

1 )  The nature of lens Convex

2)  The distance of object from lens = u = - 30 cm

3)   magnification of image = $\dfrac{v}{u}$

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

 Or,                    m = $\dfrac{60}{-30}$

∴                         m = - 2

So, The magnification = m = - 2

Hence, The nature of lens is Convex , inverted , same size of object

The distance of object from lens is - 30 cm

The magnification of image is - 2   . Answer

Answer 2

Given :

Focal length of lens =f=20cm

Image distance = 60cm

Object distace=u=?

from lens formula :

1/f=1/v-1/u

1/u=1/v-1/f

=1/60 - 1/20

=1-3/60

=-2/60

=-1/30

u= - 30cm

1) Identify the lens :

lens is Convex

2.The lens is present infront of the object at a distance of - 30cm

3. magnification=m=v/u

m=60/-30

m=-2

It indicates that image is

• real

• inverted

• enlarged

And object is kept between F and C and image is formed beyond C