Simple Nature - Light and Matter

j / In the mirror image, theareas of positive excess trafficdensity are still positive, butthe velocities of the cars haveall been reversed, so areas ofpositive excess velocity havebeen turned into negative ones.k / A pulse encounters twoboundaries.l / A sine wave has been reflectedat two different boundaries, andthe two reflections interfere.The freeway traffic wave is in fact a good model of a sound wave,and a sound wave can likewise be described either by the density(or pressure) of the air or by its speed. Likewise many other typesof waves can be described by either of two functions, one of whichis often the derivative of the other with respect to position.Now let’s consider reflections. If we observe the freeway wave ina mirror, the high-density area will still appear high in density, butvelocity in the opposite direction will now be described by a negativenumber. A person observing the mirror image will draw thesame density graph, but the velocity graph will be flipped across thex axis, and its original region of negative slope will now have positiveslope. Although I don’t know any physical situation that wouldcorrespond to the reflection of a traffic wave, we can immediately applythe same reasoning to sound waves, which often do get reflected,and determine that a reflection can either be density-inverting andvelocity-uninverting or density-uninverting and velocity-inverting.This same type of situation will occur over and over as one encountersnew types of waves, and to apply the analogy we needonly determine which quantities, like velocity, become negated in amirror image and which, like density, stay the same.A light wave, for instance, consists of a traveling pattern of electricand magnetic fields. All you need to know in order to analyze thereflection of light waves is how electric and magnetic fields behaveunder reflection; you don’t need to know any of the detailed physicsof electricity and magnetism. An electric field can be detected, forexample, by the way one’s hair stands on end. The direction of thehair indicates the direction of the electric field. In a mirror image,the hair points the other way, so the electric field is apparently reversedin a mirror image. The behavior of magnetic fields, however,is a little tricky. The magnetic properties of a bar magnet, for instance,are caused by the aligned rotation of the outermost orbitingelectrons of the atoms. In a mirror image, the direction of rotationis reversed, say from clockwise to counterclockwise, and so the magneticfield is reversed twice: once simply because the whole pictureis flipped and once because of the reversed rotation of the electrons.In other words, magnetic fields do not reverse themselves in a mirrorimage. We can thus predict that there will be two possible types ofreflection of light waves. In one, the electric field is inverted and themagnetic field uninverted (example 23, p. 699). In the other, theelectric field is uninverted and the magnetic field inverted.6.2.3 Interference effects6.2.4 Interference effects6.2.5 Interference effectsIf you look at the front of a pair of high-quality binoculars, youwill notice a greenish-blue coating on the lenses. This is advertised368 Chapter 6 Waves