How to Ascertain the Optical Property of Lens

In physics teaching,most students can't understand the optical property clearly.In the essay,a distinct way to judge the optical property of lens is made from the perspective of theory.If the light propagates from left to right in the principle of united symbols,the optical property of lens can be judged through the plus or the minus of the image space.If f'0,the lens is a convergent lens;if f' 0,it is a a divergent lens.