Hi, I am Liu, a undergraduate in mainland China, I post out this idea and want to see how you gentleman think, here it is: Griffiths says, in his book < Introduction to Quantum Mechanics >, if an insect fly in a path perpendicular to the direction in which lightbeam goes, the velocity of its shadow is proportional to the distance between light source and shadow. I think the problem is, the velocity of insect's shadow is same as insect's v. From perspective of wave theory, you can draw a picture below: －－Ｇｏａｌ ｐｌａｎｅ－－－ ｜｜｜＼ ＼｜｜｜｜｜｜｜｜｜ ｜｜｜｜＼ ＼｜｜｜｜｜｜｜｜ ｜｜｜｜｜＼ ＼｜｜｜｜｜｜｜ ｜｜２｜｜｜＼１＼｜｜｜２｜｜ ｜｜｜｜｜｜｜＼ ＼｜｜｜｜｜ ｜｜｜｜｜｜｜｜＼ ＼｜｜｜｜ ｜｜｜｜｜｜｜｜｜｜●－＞｜｜ Ｖ Two parts of the space is divided, 1 is for in which light doesn't exist, opposite the 2, and ● is the insect. Now think of the move of "slash tunnel", it is like "be pushed by light", hence the end of the tunnel, or the shadow, is moving at the insect's speed on the goal plane. Am I right? Sorry for grammar error, if any.