Question

A point object is placed at a distance of 12 cm from a convex lens on its principal axis. Its image is
formed on the other side of the lens at a distance of 18 cm from the lens. Find the focal length of the
lens.​

Answer 1

Let the object is placed on the right side of lens.

so the image is formed on the left side ..

APPLYING LENS FORMULA

1/V -1/U = 1/F

If the object is at the right side .

so the u is negative ..

1/18 -(-1/12) = 1/f

Taking LCM

( 2 + 3 )/ 18 = 1/f

18 /5 = f