A short linear object of length l lies along the axis of a concave mirror of focal length f at a distance u from the pole of the minor. The size of the image is approximately equal to

From  mirror  formula1f=1v+1u      .   .   .   .   .  iDifferentiating  equation i,  we  obtain0=−1v2dv−1u2du⇒dv=−vu2du   .   .   .   iiAlso  from  equationivu=fu−f   .   .   .   .   .   .  iiiFrom  equations  ii  and  iii  we  getdv=−fu−f2.lTherefore  size   of  image  is  fu−f2l