A convex lens of power +6 dipotre is placed in contact with a concave lens of power −4 dioptre. What will be the nature and focal length of this combination?

Power of combination = (6−4)D=2D Power =100f(in cm) 2=100f(in  cm) Or f(in cm)=50 Since the net power is positive, therefore the combination shall behave like a convex lens.