7 - The wave-particle duality

Summary

It is clear that the X-rays thus scattered proceed in direct quanta of radiant energy; in other words, that they act as photon particles. … Time does not permit me to review the evidence that was accumulating in the meantime that gave full support likewise to the electromagnetic wave character of the X-rays. … It became evident that though X-rays moved and did things as particles, they nevertheless have also the characteristic optical qualities that identify them as waves.

A. H. Compton, 1961, recalling the events of the decade 1915–25.

Hertz revisited

In chapters 5 and 6, we noted that Heinrich Hertz discovered the photoelectric effect while investigating electromagnetic waves. A strong spark produces not only visible light but also ultraviolet light. When ultraviolet light from a spark in Hertz's transmitter circuitry struck the metal of his detector, the ultraviolet ejected some electrons. Those electrons, now present in the gap across which a spark was to jump, made it easier for a spark to jump. After all, a spark is an electric current, and the presence of the ejected electrons gave the spark a head start (for air is ordinarily a poor conductor of electricity, having few mobile charged particles). What Hertz perceived directly was that, when ultraviolet light shone on the detector, the spark was easier to start and could be made to jump a larger gap.

This was fascinating, and Hertz did spend six months establishing conclusively the essential role of ultraviolet light, but then he returned to electromagnetic waves.