You will learn that light has aspects of both waves and particles. We will introduce three models of light.

• The wave model of light—the subject of this chapter—describes how light waves spread out and how the superposition of multiple light waves causes interference.

• The ray model of light, in which light travels in straight lines, will explain how mirrors and lenses work. It is the subject of Chapter 34.

• The photon model of light, which will be discussed in Part VIII, is an important part of quantum physics.

One of our tasks will be to learn when each model is appropriate.

《 LOOKING BACK Sections 16.5, 16.7, and 16.8 Light waves, wave fronts, phase, and intensity

Diffraction is the ability of a wave to spread out after passing through a small hole or going around a corner. The diffraction of light indicates that light is a wave. One interesting finding will be that a smaller hole causes more spreading.

Yes. You previously studied the thin-film interference of light reflecting from two surfaces. In this chapter we will examine the interference fringes that are seen after light passes through two narrow, closely spaced slits in an opaque screen.

《 LOOKING BACK Section 17.7 Interference

A diffraction grating is a periodic array of closely spaced slits or grooves. Different wavelengths are sent in different directions when light passes through a diffraction grating. Two similar wavelengths can be distinguished because the fringes of each are very narrow and precisely located.

Diffraction gratings are the basis of spectroscopy, a tool for analyzing the composition of materials by the wavelengths they emit. Interferometers make precise measurements, ranging from the vibrations of wings to the movements of continents, with the controlled use of interference. And interference plays a key role in optical computers.