33.1.1 Three Views

Light is a real physical entity, but the nature of light is elusive. Light is the chameleon of the physical world. Under some circumstances, light acts like particles traveling in straight lines. But change the circumstances, and light shows the same kinds of wave-like behavior as sound waves or water waves. Change the circumstances yet again, and light exhibits behavior that is neither wave-like nor particle-like but has characteristics of both.

Rather than an all-encompassing “theory of light,” it will be better to develop three models of light. Each model successfully explains the behavior of light within a certain domain—that is, within a certain range of physical situations. Our task will be twofold:

To develop clear and distinct models of light.
To learn the conditions and circumstances for which each model is valid.

We’ll begin with a brief summary of all three models.

Three models of light

The Wave Model

The wave model of light is responsible for the well-known “fact” that light is a wave. Indeed, under many circumstances light exhibits the same behavior as sound or water waves. Lasers and electro-optical devices are best described by the wave model of light. Some aspects of the wave model were introduced in Chapters 16 and 17, and it is the primary focus of this chapter.

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The Ray Model

An equally well-known “fact” is that light travels in straight lines. These straight-line paths are called light rays. The properties of prisms, mirrors, and lenses are best understood in terms of light rays. For the most part, waves and rays are mutually exclusive models of light. One of our important tasks will be to learn when each model is appropriate. Ray optics is the subject of Chapters 34 and 35.

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The Photon Model

Modern technology is increasingly reliant on quantum physics. In the quantum world, light behaves like neither a wave nor a particle. Instead, light consists of photons that have both wave-like and particle-like properties. Much of the quantum theory of light is beyond the scope of this textbook, but we will take a peek at some of the important ideas in Part VIII.

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