Question

At what distance should an object be placed from a convex length of focal length 18cm to obtain an image at 36cm from it

Answer 1

Heya

We can use the lens formulae here

Image distance = 36cm

Focal length = 18cm

1/f = 1/v - 1/u

Using sign convention

1/18 = 1/36 - 1/u

1/18 - 1/36 = 1/u

(2-1)/36 = 1/u

1/36 = 1/u

u=36m

The object should be placed at 36cm