CHAPTER 19: THE PHOTOELECTRIC EFFECT

How do solar panels generate electricity? How do automatic doors sense your approach? Why does ultraviolet light cause metals to lose electrons but red light does not, no matter how bright? The answer lies in the photoelectric effect—the phenomenon where light ejects electrons from a metal surface. This effect could not be explained by classical wave theory and led Albert Einstein to propose that light consists of particles (photons), each carrying a discrete amount of energy. This chapter will give you mastery over the photoelectric effect, Einstein's equation, and its practical applications.

19.1 INTRODUCTION TO THE PHOTOELECTRIC EFFECT

19.1.1 Definition

The photoelectric effect is the emission of electrons from a metal surface when electromagnetic radiation (light) of sufficient frequency shines on it.

The emitted electrons are called photoelectrons.

19.1.2 Historical Context

The photoelectric effect was first observed by Heinrich Hertz in 1887 during his experiments with electromagnetic waves. He noticed that a spark jumped more easily between electrodes when ultraviolet light shone on them. Later experiments by Philipp Lenard showed that light could eject electrons from metal surfaces.

The phenomenon could not be explained by the classical wave theory of light, which predicted that:

• Increasing light intensity should increase the kinetic energy of emitted electrons (it doesn't—it increases the number).
• The