Question

At what distance from a convex lens should an object of height 15 cm
be placed to obtain an image of height 45 cm. The image is formed 60 cm
behind the lens. Also calculate the focal length of the lens.​

Answer 1

Answer:

For convex lens sign convention is-

V=+ve

a=−ve

f=+ve

I (length of image) =−ve (real image)

O (size of object) =+ve

So, using lens formula

−

+O

−I

=

u

+V

⇒u=−

3

v

⇒v=−3u

v

1

−

u

1

=

f

1

⇒

−3u

1

−

u

1

=

f

1

⇒

3u

4

=−

15

1

⇒u=−20cm

object is placed 20cm in front of the lens