This chapter studies wave interference. Light does not bend in a lens, it does not bounce off the surface of glass, and it does not spread out after passing through a small hole. All of these are illusions. It only appears that light bends, bounces, and spreads out. The effects that people see—reflection, refraction, colors in soap bubbles—come from the fact that photons interact with one another in an unusual way. The interaction is often referred to as wave interference because it can be described mathematically much like the interference of water waves, but the reality of it is considerably stranger. The strangeness is discussed in more detail in the last chapter, but a hint of it can be seen from the fact that photons appear to interfere with one another even if they are emitted one at a time, which water waves obviously do not do.
This chapter studies wave interference. Light does not bend in a lens, it does not bounce off the surface of glass, and it does not spread out after passing through a small hole. All of these are illusions. It only appears that light bends, bounces, and spreads out. The effects that people see—reflection, refraction, colors in soap bubbles—come from the fact that photons interact with one another in an unusual way. The interaction is often referred to as wave interference because it can be described mathematically much like the interference of water waves, but the reality of it is considerably stranger. The strangeness is discussed in more detail in the last chapter, but a hint of it can be seen from the fact that photons appear to interfere with one another even if they are emitted one at a time, which water waves obviously do not do.
This chapter discusses scattering. As mentioned already, optics is primarily photons interacting with electrons, atoms, and molecules. If the energy of the photon matches the difference in energy between two levels of excitation, then the photon vanishes, its energy converted into other forms. However, if the photon's energy does not find a match among the differences in possible energy states, a new photon will quickly be emitted. The new photon is usually the same energy—and thus the same wavelength—as the old one, making it appear as if the original photon bounced, which is why this nonabsorptive interaction is known as “scattering.” When the energy of the new photon matches that of the old one, the scattering is known as elastic. When the energy of the new photon is lower, the scattering is known as inelastic. All the observed phenomena of scattering—refraction, reflection, structural colors, turbidity, and more—are based on these few principles.
In making some experiments on the fringes of colors accompanying shadows, I have found so simple and so demonstrative a proof of the general law of the interference of two portions of light, which I have already endeavored to establish, that I think it right to lay before the Royal Society a short statement of the facts which appear to me so decisive. Thomas Young, Bakerian Lecture, November 24, 1803 Interference defined In chapter 3, we framed the question, can we account for the behavior of light by supposing that light is wave motion of some kind? So far, the logic of our response has been of this form: we asked experimentally, do waves have property X that we know light possesses? For “property X ,” we took the existence of reflection the equality of the angles of reflection and incidence the existence of refraction, and Snell's law, in the sense that, in refraction, the ratio of sines is independent of the angle of incidence. Each time, we could answer, yes, waves do have this property X that we know light possesses. Beyond these tests, we found that the wave theory's prediction for the index of refraction – namely, a specific ratio of wave speeds – was resoundingly confirmed by Michelson's measurements of. Things are going well for a wave theory of light. The time has come to shift the perspective and to change the logic of the testing.
Waves of light Light behaves like a wave. In the laboratory, we can measure the vibrations that are set up in matter when a light wave comes by. We can also observe the peculiar light-and-dark patterns that occur when two light rays are made to act simultaneously, an unmistakable sign of wave behaviour. Let us make a small excursion into wave motion. Throughout this exposition, you are encouraged to experiment as much as possible with waves (preferably real ones, or at least those in the diagrams), to become familiar with their fascinating behaviour. Water waves in the bathtub, or those seen on open water from a high tower or an airplane, are especially instructive. Waves are periodic; if you pick a point in space (e.g. the surface of a pond at the point where a reed pokes through) and you watch the motion of the wave carrier (the water that bobs up and down), then you will see that the state of motion of the carrier repeats itself at regular intervals of time. This interval is the period of the wave. A free wave has a velocity, i.e. a direction and a speed. At a given point, a wave alternates periodically between a certain maximum and a minimum.
How can we construct a picture of the photon from the wealth of observation material available to us? The photon appears to have a split personality: it is neither a wave nor a particle but something else which, depending on the experimental situation, exhibits a wave- or a particle-like behavior. In other words, in the photon (as in material particles such as the electron) the particle–wave dualism becomes manifest. Whereas classically the wave and the particle pictures are separate, quantum mechanics accomplishes a formal synthesis through a unified mathematical treatment. Let us look first at the wave aspect familiar from classical electrodynamics, which seems to be the most natural description. It makes all the different interference phenomena understandable, such as the “interference of the photon with itself” on the one hand and the appearance of spatial and temporal intensity correlations in a thermal radiation field on the other (which are obviously brought about by superposition of elementary waves emitted independently from different atoms). It might come as a surprise (at least for those having quantum mechanical preconceptions) that the classical theory is valid down to arbitrarily small intensities: the visibility of the interference pattern does not deteriorate even for very small intensities – the zero point fluctuations of the electromagnetic field advocated by quantum mechanics do not have a disturbing effect – and is valid not only for conventional interference experiments but also for interference between independently generated light beams (in the form of laser light).
The previous chapter surveyed part of the rich variety of physical phenomena that can be understood using the ideas of quantum physics. Now that we are beginning the task of looking more deeply into the subject we shall find it useful to concentrate on examples that are comparatively simple to understand but which still illustrate the fundamental principles and highlight the basic conceptual problems. Some years ago most writers discussing such topics would probably have turned to the example of a ‘particle’ passing through a two-slit apparatus (as in Figure 1.2), whose wave properties are revealed in the interference pattern. Much of the discussion would have been in terms of wave–particle duality and the problems involved in position and momentum measurements, as in the discussion of the uncertainty principle in the last chapter. However, there are essentially an infinite number of places where the particle can be and an infinite number of possible values of its momentum, and this complicates the discussion considerably. We can illustrate all the points of principle we want to discuss by considering situations where a measurement has only a small number of possible outcomes. One such quantity relating to the physics of light beams and photons is known as polarisation . In the next section we discuss it in the context of the classical wave theory of light, and the rest of the chapter extends the concept to situations where the photon nature of light is important.
This concluding chapter explains that the modern theory of light falls within the field of quantum mechanics. At first glance, quantum mechanics does not seem that strange—its name is based on the fact that light comes in units and that electrons have discrete energy states. It also includes the uncertainty principle, which states that one cannot know certain pairs of physical properties with perfect precision. Moreover, quantum mechanics involves the wave-particle duality of photons. The chapter then explores two of the most unusual aspects of quantum mechanics: two-slit interference and quantum entanglement. Both violate the most fundamental notions about how the world works.