Wave-Particle Duality
In classical physics, the concepts of waves and particles are mutually exclusive. A
classical particle behaves like a piece of shot. It can be localized and scattered, it exchanges energy suddenly at a point in space, and it obeys the laws of conservation of energy and momentum in collisions. It does
not exhibit interference or diffraction. A
classical wave, on the other hand, behaves like a sound or light wave. It exhibits diffraction and interference, and its energy is spread out continuously in space and time. Nothing can be both a classical particle and a classical wave at the same time.
We ordinarily think of light as a wave, but light exhibits particle properties when it interacts with matter, as in the photoelectric effect. Electrons, which we usually think of as particles, exhibit the wave properties of interference and diffraction when they pass near the edges of obstacles. All carriers of momentum and energy (for example, electrons, atoms, or photons) exhibit both wave and particle characteristics. Everything propagates like a wave and exchanges energy like a particle.
Often the concepts of the classical particle and the classical wave give the same results. If the wavelength is very small, diffraction effects are negligible, so the waves travel in straight lines like classical particles. Also, interference is not seen for waves of very short wavelength, because the interference fringes are too closely spaced to be observed. It then makes no difference which concept we use. If diffraction is negigible, we can think of light as a wave propagting along rays, as in geometrical optics, or as a beam of photon particles. Similarly, we can think of an electron as a wave propagating in straight lines along rays or, more commonly, as a particle.
We can also use either the wave or particle concept to describe exchanges of energy if we have a large number of particles and we are interested only in the average values of energy and momentum exchanges.
Excerpted and adapted from: Tipler, Paul A., and Gene Mosca. 2008. Physics for Scientists and Engineers. W. H. Freeman and Company, NY. 6th Edition, p 1187.