We combine two lenses, one is convex and other is concave having focal lengths f1 and f2 and their combined focal length is F. In what condition the combination of the lenses will behave like concave lens?

f1 > f2 f1 = f2 f1 < f2 f1 >= f2

f1 > f2

Focal length of the combination = \(\frac{f1 . f2}{f1 + f2}\)