Question

An an image 2 by 3 of the size of the object is formed by a convex lens at the distance of 12 cm from it .find the focal length of the lens.

Answer 1

good morning.

as per the question ,

magnification ( m ) =  2/3  ,    object distance ( u ) =   - 12cm

now we have to take m as -ve because in convex lens the case in which the image size is less than the object then the image is also real and inverted.

m = v/u

- 2/3 = v/-12

v= 2 × 12/3

v= 8cm

now using the lens formula

1/v - 1/u = 1/f

1/8 + 1/12 = 1/f   [ u = - 12 ]

taking the lcm

3 + 2 / 24 = 1/f

5/24 = 1/f

f = 24/5 cm = 4.8 cm.

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