III. History Philosophers from as far back in history as the Greeks of the 5th century BC have thought about the nature of light. In the 1600s, scientists began to argue over whether light is made of particles or waves. In the 1860s, British physicist James Clerk Maxwell discovered electromagnetic waves, waves of electromagnetic energy that travel at the speed of light. He determined that light is made of these waves, and his theory seemed to settle the wave versus particle issue. His conclusion that light is made of waves is still valid. However, in 1900 German physicist Max Planck renewed the argument that light could also act like particles, and these particles became known as photons. He developed the idea of photons to explain why substances, when heated to higher and higher temperatures, would glow with light of different colors. The wave theory could not explain why the colors changed with temperature changes. Most scientists did not pay attention to Planck's theory until 1905, when German-born American physicist Albert Einstein used the idea of photons to explain an interaction he had studied called the photoelectric effect. In this interaction, light shining on the surface of a metal causes the metal to emit electrons. Electrons escape the metal by absorbing energy from the light. Einstein showed that light behaves as particles in this situation. If the light behaved like waves, each electron could absorb many light waves and gain more and more energy. He found, however, that a more intense beam of light, with more light waves, did not give each electron more energy. Instead, more light caused the metal to release more electrons, each of which had the same amount of energy. Each electron had to be absorbing a small piece of the light beam, or a particle of light, and all these pieces had the same amount of energy. A beam of light with a higher frequency contained pieces of light with more energy, so when electrons absorbed these particles, they too had more energy. This could only be explained using the photon view of radiation, in which each electron absorbs a single photon and gains enough energy to escape the metal. Today scientists believe that light behaves both as a wave and as a particle. Scientists detect photons as discrete particles, and photons interact with matter as particles. However, light travels in the form of waves. Some experiments reveal the wave properties of light; for example, in diffraction, light spreads out from a small opening in waves, much like waves of water would behave. Other experiments, such as Einstein's study of the photoelectric effect, reveal light's particle properties.