Question

A Lens forms an image of height 3 cm of an object of height 1 cm. The distance between the image and object is 15 cm. What is the focal length of the lens?

Answer 1

Height of image/height of object= -v/u

1/u+ 1/v=1/f

1-1/3=1/f

F= 1.5cm

Answer 2

Let u be the object distance, v be the image distance and f be the focal length.

So apq,

M (magnification)= -3

So the image is real.

Therefore,

v/u= -3

=> v= -3u

Also,

-u + v= 15

=> -u -3u= 15

=> -4u= 15

=> u= -15/4

So v= -3 x -15/4= 45/4

We know that,

1/f= 1/v- 1/u

  = 4/45 + 4/15 ( reciprocal)

  = 2.8cm

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