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?

答案:E
統計:A:0,B:0,C:0,D:0,E:0
難度:計算中