The incident ray from object when incidents at the first surface undergoes refraction. If there is no second surface, then the position of image in medium n₂ will be given as

                                                                  ------------------(A)

The image formed by refraction at first surface will act as object for the refraction at second surface. If 'd' is thickness of lens, . (-ve sign because the object I₁ is on the right of vertex of surface 2 the final image I will be formed at v

                                                                  ------------------(B)

         or