Further we do not have a simple lens
equation true for all u (and v) in terms of a definite constant of the
system  (the constant being determined by f1 and f2, and the separation
between the lenses) The notion of effective focal length, therefore,
does not seem to be meaningful for this system (b)
u1 = – 40 cm, f1 = 30 cm, gives v1= 120 cm Magnitude of magnification due to the first (convex) lens is 3