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Cambridge University Press eBooks

Light

Vincent Icke

Published Feb 2, 1995• 0 citations
Publisher

Abstract

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.

DOI
10.1017/cbo9780511622694.005

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Cambridge University Press eBooks•Oct 23, 2003•2 citations
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.
Neither particle nor wave The information about the particle behaviour and the wave behaviour of light, presented in the preceding chapter, forms an extremely peculiar combination when taken together. And yet both are true: the experiments have been done as described, with the outcome as stated. There are experiments that tell us clearly that light consists of waves; there are others that tell us, equally clearly, that light consists of particles. Somehow we must find a description of light that incorporates both kinds of behaviour. This is a formidable undertaking, but there is one cheerful fact: in absolutely none of the experiments that have ever been done did the particle and the wave behaviours occur simultaneously . Because this is a fact that covers the entire history of physics, we feel justified in assuming that this is a general feature of Nature, and that there cannot ever be an experiment in which the wave and particle behaviours both occur at the same time. This is a generalization, for we can never expect to do all possible experiments. This generalizing hypothesis was first announced by Bohr, who had the wonderful combination of daring and insight to postulate that the observation of certain quantities excludes the simultaneous observation of certain others.
Wave motion exists everywhere. When a bird sings in the forest, sound waves propagate away from the bird’s throat in all directions, striking the ear membranes of the other animals nearby. A leaf falling from a tree on the surface of a pond produces ripples that spread outward through the surface of the serene water. Light reaching the surface of the earth from the sun propagates through empty space as an electromagnetic wave. And as we learned in Chapter 7, even ordinary matter exhibits wave behavior.
Hands-on exercise: wave and particle properties The purpose of this exercise is for you to observe some basic wave and particle properties. To complete this exercise you will need the following: Tub of water, or access to a quiet pond, lake or swimming pool. Things to float on the surface of the water. Pen or pencil and some drawing paper. Small projectile such as a stone. Disturb the middle of the tub just until you are able to make a visible wave on the surface. Watch how the wave propagates. Wait until the surface of the water returns to being flat and make another wave. Keep doing this as many times as necessary to be able to draw what you see on the paper and answer the following questions: Does the wave have a definite location at any one moment in time? Does the wave have a definite direction as it propagates? Approximately how far does the wave travel in 1 s? Describe the motion of the water in which the wave moves. Throw your small projectile in the air at various angles, letting it drop down (not in the water). Keep doing this as many times as necessary to be able to draw what you see and answer the following questions: Does the object have a definite location at any one moment in time? […]
The double-slit experiment is usually taken as evidence that a particle is also a wave. We show by simulation that a definite particle, moving through a physical medium and guided by its own wave, produces the same interference pattern, with no constant fitted to the pattern. Space is modeled as a compressible particle medium with a short-range stiffness. A particle set in motion disturbs it once. The disturbance disperses, and the particle carries with it the part of the wave whose group velocity matches its own speed, so the wavelength at the particle is fixed by its speed alone, λ ∝ 1/v, with no oscillator and no continued pushing. That wave passes through two slits and interferes as any wave does. The particle passes through one slit, keeps its speed, and takes its direction from the flow of the wave around it. It lands where the wave is bright. Hundreds of particles, sent one at a time, reproduce the wave's own pattern: at three speeds and two slit spacings, up to nine bright lanes with empty dark bands between them, lane spacing that scales with the wavelength of the wave it carries, the same pattern for a stream of particles as for one, and no lanes when one slit is closed. The rule that steers the particle has no strength, no frequency, and no free constant, and it is the same rule at every scale because each particle's wave is sized to itself. The result depends on one condition: that the particle's launch point lie well beyond the near-field distance of the slit pair, which laboratory sources satisfy. We record the rules that were tried and failed and the numerical pitfalls, and we state what remains open: the silent coast is a postulate, the mass dependence of the wavelength is untested, and the work is two-dimensional.
… there are certain situations in which the peculiarities of quantum mechanics can come out in a special way on a large scale. Richard Feynman Laser light Nowadays, everyone has heard of lasers, and laser light displays are a frequent ingredient of modern rock concerts. Laser light has many applications, ranging from astronomy to hydrogen fusion. What is the special feature of laser light that makes it so useful? The answer to this question involves a property of wave motion known as ‘coherence’, with light photons acting together in a special form of quantum mechanical co-operation. This type of quantum co-operation will turn out to be vital for an understanding of the peculiar behaviour of quantum ‘superfluids’. To understand the special nature of laser light, however, we must first explain what is meant by coherence. Consider the simple wave motion shown in Fig. 7.1. We see that the pattern repeats itself after one wavelength and the frequency of the wave corresponds to the number of wavelengths sent out per second. If this wave is a wave on a string, each point on the string just moves up and down with a certain amplitude: the maximum distance the point can move out to before it starts to come back. Up to now, this is really all we have needed to know about waves. Now consider two waves of the same wavelength but started at slightly different times, as shown in Fig. 7.2.