IV.

Quanta of Radiation

At the beginning of the 20th century, however, physicists found that the wave theory did not account for all the properties of radiation. In 1900 the German physicist Max Planck demonstrated that the emission and absorption of radiation occur in finite units of energy, known as quanta. In 1905, Albert Einstein was able to explain some puzzling experimental results concerning the photoelectric effect by suggesting that electromagnetic radiation can behave like a particle.

Other phenomena that occur in the interaction between radiation and matter can also be explained only by the quantum theory. Thus, modern physicists were forced to recognize that electromagnetic radiation can behave sometimes like a particle and sometimes like a wave. The parallel concept—that matter also exhibits particle-like and wave-like characteristics—was developed in 1925 by the French physicist Louis de Broglie. SeeWave-Particle Duality.