6 - The photon

Summary

There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.

Mark Twain, Life on the Mississippi

Preview

The last three chapters developed a wave theory of light. Indeed, they suggested that light is an electromagnetic wave, and then they marshaled experimental support for that view. The wave theory of light is correct – but it is not the whole story. Some other idea complements that theory. In this chapter, new experiments compel us to develop the alternative idea, and in chapter 7 we learn how the wave theory and the new idea complement each other.

The photoelectric effect

In this day and age, we are accustomed to the idea that sunlight is a source of energy. Let us investigate the connection between light (in general) and energy.

If you are not thoroughly familiar with how the word “energy” is used in physics, now is the right time to read appendix A, Energy. Knowing what physicists mean with their words will help you to understand the line of reasoning.

Electron escape

When an electron absorbs light, it acquires additional energy. If the electron is in a piece of metal, it may be able to escape from the metal. The positive charges in the metal tend to hold the electrons back and prevent their escape. (To be sure, the electrons can move about quite freely inside a good electrical conductor.)