Question

size of image of an object by a lens having focal length of 20cm is observed to be magnified towards 4 third of its size what is the nature of the image and the lens at what distance the object has been placed from the lens

Answer 1

Answer:

Step-by-step explanation:

Given Size of image of an object by a lens having focal length of 20cm is observed to be magnified towards 4 third of its size what is the nature of the image and the lens at what distance the object has been placed from the lens

Given f = 20 cm

Magnification = m = v/u

                        = 4/3 = v/u

           So 3 v = 4 u

So v = 4 u / 3

We know that 1/f = 1/v + 1/u

                     1/f = 3/4u + 1/u

                     7u/4u^2

1/f = 7/4u

4u = 7f

Or u = 7f / 4

     So u = 7/4 (20)

Or u = 35 cm

So object distance is 35 cm

Now 1/v = 1/20 - 1/35

1/v = 3/140

So v = 140/3

Now image is a virtual image.