4. Two thin lenses form a compound lens ..-阿摩線上測驗
4. Two thin lenses form a compound lens system. The front focal length (FFL) of this
compound lens system is defined as the distance between a point source and the front
lens so that the light passing this system will become parallel. In the figure, the two
lenses have focal length f1 = 10 cm and f2 = -10 cm, respectively, and are separated by
a distance d = 5 cm. What is the FFL of this compound lens system?
(A) 5 cm
(B) 10 cm
(C) 12.5cm
(D) 25 cm
(E) 30 cm