✔ 最佳答案
The principal foci have already been marked on the digrams. They are the points marked F and F'
2011-12-17 17:04:25 補充：
Please refer to the following web-page for explanation of the principal foci,
http://www.sasked.gov.sk.ca/docs/physics/u6b2phy.html
2011-12-18 01:51:55 補充：
Just read the para. extracted from the given web-page:
The principal focus (F) is a point on the principal axis where light comes to a focus (for a converging lens) or appears to be diverging from (for a diverging lens)...
2011-12-18 01:52:34 補充：
...Two foci exist, equidistant on either side of the lens, since light behaves the same way when travelling in either direction (Principle of Reversibility). The two foci, F and F' are called the primary principal focus and the secondary principal focus, respectively....
2011-12-18 01:52:48 補充：
... F, sometimes also referred to as the primary focal point, is shown on the right side of a converging lens, and on the left side of a diverging lens, while F', the secondary focal point is shown on the opposite side of each respective lens.)
