根据“费马 (Ｆｅｒｍａｔ)原理”我们知道 :光在传播过程中“走”的是时间最短的路径 .依此我们可以构建合适的模型 ,巧用类比 ,触发联想 ,解决一类运动学问题 .例 1 .如果树上停着一只乌鸦 ,而地上有几颗麦粒 .那么 ,乌鸦从树上飞下来吃麦粒 ,再飞到对面篱笆上的话 ,乌鸦须落 According to the “Fermat principle,” we know that light is the “shortest path” in the process of propagation. Therefore, we can build suitable models, use analogies, trigger associations, and solve a class of kinematic problems. Example 1. If a crow is parked on a tree and there are several grains on the ground, then the crow flies down from the tree to eat the grain, then flies to the opposite fence, and the crow will fall.