圖片參考:http://imgcld.yimg.com/8/n/HA08066696/o/701112170046013873412330.jpg
1. convex lens: where is principal foci?
圖片參考:http://imgcld.yimg.com/8/n/HA08066696/o/701112170046013873412341.jpg
2. concave lens: where is principal foci?
更新1:
感係咪姐話F'都可以叫principal focus?
更新2:
principal focus同focus有咩分別?
更新3:
thx very much