The following figure from shows version of a zoom.The converging lens has a focal length `f_(1)`and the diverging lens has focal length `f_(2)=-|f_(2)|`.the two lens are separated by a variable distance d that it is always less than `f_(1),`also the magnitude of the focal length of the diverging lens satisfies the inequality `|f_(2)|gt(f_(1)-d).` if the rays that emerge form the diverging lens and reach the final image point are extended backward to the left of the diverging lens,they will eventually expand to the original radius `r_(o)`at the same point.`Q`.To determine the effective focal length of the combination lens consider a bundle of parallel rays of radius `r_(0)`entering the emerging lens. the effective focal length `f` is given byA. `(f_(1)|f_(2)|)/(|f_(2)|-f_(1)+d)`B. `(f_(1)f_(2))/(f_(1)-f_(2)-d)`C. `(f_(1))/(f_(2)-f_(1)+d)`D. `(f_(2))/(f_(1)+f_(2)+d)`