Question

A convex lens forms a virtual image when an object is placed at a distance of 18cm from it. the local length must be

Answer 1

greater than 18 cm ........ .........focal length.
as only image distance is given accurate measurements is not possible.
a convex lens forms virtual image only when the object is kept be focal length and pole

Answer 2

Answer:

If a convex lens forms a virtual image when an object is placed at a distance of 18cm from it, then the local length must be greater than 18.

Explanation:

The virtual image produced by a convex lens when the source is closer to the lens than the focal length, f.

For virtual image v < 0

u = -18 cm, f> 0

1/v - 1/u = 1/f

1/v - (1/-18) = 1/f

1/v + 1/18 = 1/f

1/v = 1/f - 1/18

1/v = (18 -f)/18f

v = 18f/(18-f)

For v< 0

So, f >18

So, the local length must be greater than 18.