(a) It is the central point of the lens through which a ray of light passes without suffering any deviation.
(b) The object will be placed at F.
© f = - 20 cm, u = ?, v = - 15 cm, h = ?
Using the expression
1/f = 1/v - 1/u , we have
1/-20 = 1/-15 - 1/u
u = 60 cm