Question

find distance between two possible position of a real object in front of a convex lens of focal length 20 CM such that the size of image is the triple of size of object..

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Answer 1

40/3 .................

Answer 2

Given:

Focal length of the lens = 20cm

Magnification of the lens = Three times =  - 3 ( real object)

To find:

The distance between the position of a real object.

Solution:

Magnification of lens is given as = -v/u

Thus,

v = 3u

Mirror formula = 1/f = 1/u + 1/v

1/ -20 = 1/u + 1/3u

1/-20 = 4/3u

3u/4 = -20

3u = -80

u = -80/3

= - 26.67

Now, v = - 26.67 × - 3

= - 80

Answer: The distance between the position of a real object is - 80 cm