Understanding Lasers

Chapter 2: Physical Basics

ABOUT THIS CHAPTER

Lasers evolved from concepts of modern physics that emerged early in the twentieth century. To understand lasers, you need to understand the basic concepts that underlie laser physics, including light, atomic energy levels, quantum mechanics, and optics. This chapter starts with the nature of light, then moves on to how light is generated, the interactions of light and matter, and some fundamentals of optics, to give you the background you need to understand lasers themselves.

2.1 ELECTROMAGNETIC WAVES AND PHOTONS

Early physicists debated long and loud over the nature of light. Isaac Newton held that light was made up of tiny particles. Christian Huygens believed light was made up of waves, vibrating up and down, perpendicular to the direction in which the light travels. Newton's theory came first, but Huygens' theory explained early experiments better, so for a long time it was assumed to be right.

Today, we know that both theories are partially right. Much of the time, light behaves like a wave. Light is called an electromagnetic wave because it consists of electric and magnetic fields perpendicular to each other, as shown in Figure 2-1. Because the electric and magnetic fields oscillate perpendicular to the direction in which the waves travel, they are called transverse waves.

At other times, light behaves like massless particles called photons. Photons are not exactly what Newton envisioned, but the

particle view is also critical to understanding light. We will move back and forth between the wave and particle views of light, but you should remember that they are just different ways to look at the same thing.

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