Question

a lens of focal length 12 cm forms an erect image three times the size of the object. the distance between the object and image will be
a) 8 cm
b) 16 cm
c) 24 cm
d) 36 cm
explain me

Answer 1

Relation for image size


image size = Focal Length X Object size  /  Lens-to-object distance

 

form the prob  focal length =12 cm

  object size  = m

  image size = 3m

  form the above formula  

  Lens-to-object distance=  Focal Length X Object size/ Image Size

  = 12 X m /3m= 4 cm

  
but

distance between object to image = distance  object to lense

  + distance lense to image

distance between object and lense is nothing but the focal length of the lense that is 12 cm

 so, distance between object to image = 12 + 4 = 16 cm

so option B.)16 is correct answer


hope it is useful

Answer 2

Heya!!
Here is your answer

Given m = 3 = -v/u
So v = 3u
Apply lens formula for convex lens
1/v = 1/u - 1/f
1 /3u = 1/ u - 1/12
1/u - 1 /3u = 1/12
2/3u = 1/12
u = 2 × 12/3
u = 8 cm
v = 3u = 24cm.
Distance between the object and the image is v - u = 24 - 8 = 16 cm.

Hope this helps..
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