Simple Nature - Light and Matter

trons that we have already used for photons.13.3.1 Electrons as wavesWe started our journey into quantum physics by studying therandom behavior of matter in radioactive decay, and then asked howrandomness could be linked to the basic laws of nature governinglight. The probability interpretation of wave-particle duality wasstrange and hard to accept, but it provided such a link. It is nownatural to ask whether the same explanation could be applied tomatter. If the fundamental building block of light, the photon, isa particle as well as a wave, is it possible that the basic units ofmatter, such as electrons, are waves as well as particles?A young French aristocrat studying physics, Louis de Broglie(pronounced “broylee”), made exactly this suggestion in his 1923Ph.D. thesis. His idea had seemed so farfetched that there wasserious doubt about whether to grant him the degree. Einstein wasasked for his opinion, and with his strong support, de Broglie gothis degree.Only two years later, American physicists C.J. Davisson and L.Germer confirmed de Broglie’s idea by accident. They had beenstudying the scattering of electrons from the surface of a sampleof nickel, made of many small crystals. (One can often see such acrystalline pattern on a brass doorknob that has been polished byrepeated handling.) An accidental explosion occurred, and whenthey put their apparatus back together they observed somethingentirely different: the scattered electrons were now creating an interferencepattern! This dramatic proof of the wave nature of mattercame about because the nickel sample had been melted by the explosionand then resolidified as a single crystal. The nickel atoms,now nicely arranged in the regular rows and columns of a crystallinelattice, were acting as the lines of a diffraction grating. The newcrystal was analogous to the type of ordinary diffraction grating inwhich the lines are etched on the surface of a mirror (a reflectiongrating) rather than the kind in which the light passes through thetransparent gaps between the lines (a transmission grating).Although we will concentrate on the wave-particle duality of electronsbecause it is important in chemistry and the physics of atoms,all the other “particles” of matter you’ve learned about show waveproperties as well. Figure a, for instance, shows a wave interferencepattern of neutrons.It might seem as though all our work was already done for us,and there would be nothing new to understand about electrons:they have the same kind of funny wave-particle duality as photons.That’s almost true, but not quite. There are some important waysin which electrons differ significantly from photons:852 Chapter 13 Quantum Physics