Simple Nature - Light and Matter

end of this book, we’ll even see that the things we’ve been callingparticles, such as electrons, are really waves! 16.1 Free Waves6.1.1 Wave motiona / Your finger makes a depressionin the surface of thewater, 1. The wave patternsstarts evolving, 2, after youremove your finger.Let’s start with an intuition-building exercise that deals withwaves in matter, since they’re easier than light waves to get yourhands on. Put your fingertip in the middle of a cup of water andthen remove it suddenly. You’ll have noticed two results that aresurprising to most people. First, the flat surface of the water doesnot simply sink uniformly to fill in the volume vacated by yourfinger. Instead, ripples spread out, and the process of flattening outoccurs over a long period of time, during which the water at thecenter vibrates above and below the normal water level. This typeof wave motion is the topic of the present section. Second, you’vefound that the ripples bounce off of the walls of the cup, in muchthe same way that a ball would bounce off of a wall. In the nextsection we discuss what happens to waves that have a boundaryaround them. Until then, we confine ourselves to wave phenomenathat can be analyzed as if the medium (e.g., the water) was infiniteand the same everywhere.It isn’t hard to understand why removing your fingertip createsripples rather than simply allowing the water to sink back downuniformly. The initial crater, a/1, left behind by your finger hassloping sides, and the water next to the crater flows downhill to fillin the hole. The water far away, on the other hand, initially hasno way of knowing what has happened, because there is no slopefor it to flow down. As the hole fills up, the rising water at thecenter gains upward momentum, and overshoots, creating a littlehill where there had been a hole originally. The area just outside ofthis region has been robbed of some of its water in order to buildthe hill, so a depressed “moat” is formed, a/2. This effect cascadesoutward, producing ripples.There are three main ways in which wave motion differs fromthe motion of objects made of matter.1. SuperpositionIf you watched the water in the cup carefully, you noticed theghostlike behavior of the reflected ripples coming back toward thecenter of the cup and the outgoing ripples that hadn’t yet been reflected:they passed right through each other. This is the first, andthe most profound, difference between wave motion and the mo-1 Speaking more carefully, I should say that every basic building block of lightand matter has both wave and particle properties.340 Chapter 6 Waves