Question

An image 2/3rd the size the object is formed by a convex lens at a distance of 12cm from it. Find the focal length of the lens.
anyone help me?

Answer 1

In this case of convex lens, the size of the object is bigger than the size of the

image .Thus we can conclude that the image formed is real and inverted and also the

image formed is on the other side.According to the sign convention for lenses the

image distance on the other side will be positive .

Therefore v=12cm

For real and inverted image the magnification will be negative.therefore m= - 2/3



m = v/u

-2/3 = 12/u

u = -18 cm

1/v - 1/u = 1/f

f = 36/5 cm

f = 7.2 cm

Therefore the focal length is 7.2 cm

Answer 2

hy                              

friend                                    

here is your answer

i hope its helps you

                                                             

→ the size of the object  is bigger then .

→ the size of the image we can conclude that .

→ the image formed is real and inverted and the image formed is on .

→ the other side according to the sign convention for lenses the image distance on the other side will be positive .

                                                                                                                                           

  v = 12

 m = v/u

=   -2 /3 = 12 / u

= u = 12 x 3  / -2

= 1 / v or  - 1 / u  

= 1 / f

= 1 / 12   -   1  / 18

= 1 / f

= 2 + 3 / 36

= 1 / f

f = 36 / 5

= 7.2 cm

                                                                                                                     

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