Optics is the area of physics concerned with the properties and applications of light, including how light interacts with matter. But what is light? This is an ancient question with no simple answer. Rather than an all-encompassing theory of light, we will develop three different models of light, each of which describes and explains the behavior of light within a certain range of physical situations.

The wave model is based on the well-known fact that light is a wave—specifically, an electromagnetic wave. This model builds on what you learned about waves in Part IV of this text. Light waves, like any wave, are diffuse, spread out through space, and exhibit superposition and interference. The wave model of light has many important applications, including

• Precision measurements.

• Optical coatings on lenses, sensors, and windows.

• Optical computing.

Another well-known fact—that light travels in straight lines—is the basis for the ray model. The ray model will allow us to understand

• Image formation with lenses and mirrors.

• Optical fibers.

• Optical instruments ranging from cameras to telescopes.

One of the most amazing optical instruments is your eye. We’ll investigate optics of the eye and learn how glasses or contact lenses can correct some defects of vision.

The photon model, part of quantum physics, is mentioned here for completeness but won’t be developed until Part VIII of this text. In the quantum world, light consists of tiny packets of energy—photons—that have both wave-like and particle-like properties. Photons will help us understand how atoms emit and absorb light.

For the most part, these three models are mutually exclusive; hence we’ll pay close attention to establishing guidelines for when each model is valid.