Study of Radiation

Radiation has been studied since ancient times. In the 3rd and 4th centuries BC, Epicurus, a Greek philosopher, wrote of particles "streaming off" the surface of objects. Euclid, a Greek mathematician, thought around the same time that an object could be seen because the eye sent out radiation.

Robert Grosseteste, an English bishop and scholar of the 13th century, regarded light as the root of all knowledge. He thought that understanding the laws governing light would unravel the laws of nature.

In the 17th century, Sir Isaac Newton, the English scientist, considered light to be composed of tiny particles, and Christiaan Huygens, the Dutch physicist, thought that light was made up of waves. Their followers and scientists argued each view for over a century until Thomas Young, a British physicist, showed early in the 19th century that light had properties similar to those of sound and water waves. Augustin Fresnel, a French physicist, provided more evidence for Youngs view a few years later. By 1850, most scientists had accepted that light consisted of waves.

In 1864, James Clerk Maxwell, a British scientist, suggested that light consisted of electromagnetic waves, and predicted the discovery of other, invisible forms of electromagnetic radiation. Heinrich R. Hertz and Wilhelm C. Roentgen, two German physicists, proved Maxwell's predictions correct. Hertz discovered radio waves in the late 1880s, and Roentgen discovered x-rays in 1895.

The discovery of radioactivity was a landmark in the study of radiation. In 1896, Antoine Henri Becquerel, a French physicist, discovered that crystals of a uranium compound darken photographic plates even when they were not exposed to light, and conjectured that uranium gave off energy in the form of radiation. Experiments conducted later by Ernest Rutherford, a New Zealand-born physicist, showed that this radiation was made up of two types of particles, which he named alpha and beta.

In 1898, Marie and Pierre Curie, both French physicists, found another material that produced radiation, and called it polonium. Also that year, along with Gustave Bemont, a French chemist, they discovered yet another material that gave off radiation, and called it radium. Rutherford showed a few years later that the process of transmutation could convert radioactive substances into new elements.

The work of Rutherford and the Curies led to great interest in the structure of the atom. Rutherford, his colleagues, and other scientists soon proved that the atom had a nucleus of high mass and positive electric charge surrounded by negatively charged electrons.

The quantum theory was another landmark in the study of radiation. Max Planck, a German physicist, analyzed radiation from hot objects and suggested in 1900 that objects could give off and take in this radiation only in packets of energy. These packets are called photons; initially, they were called quanta.

Albert Einstein, another German physicist, used Planck's theory to explain in 1905 the photoelectric effect. This is a process in which metal gives off electrons when struck by a bright beam of light. Einstein proposed that an electron could be freed from an atom in the metal by the energy supplied by a single photon. The localized manner in which photons act to produce the photoelectric effect resembles that of particles rather than of waves. Einstein's proposition led scientists again to consider the particle theory of light; now they know that radiation has features of both particles and waves.

Niels Bohr, a Danish physicist, based his explanation of the structure of the hydrogen atom in 1913 on the quantum theory. Bohr theorized that electrons can have only certain specific values of energy, and proved that atoms release photons of radiation when their electrons lose energy. In 1924, Louis de Broglie, a French physicist, hypothesized that electrons act as waves, called matter waves.

The first artificial nuclear chain reaction began the nuclear age in 1942. Enrico Fermi, a physicist born in Italy, and his co-workers at the University of Chicago produced that reaction. Many scientists have since then tried to discover applications of radioactivity and radiation instead of finding out what causes them. Nuclear weapons based on fission (the atomic bomb) and fusion (the hydrogen bomb) were developed. The first full-scale nuclear power plant began operating in 1956. Radiation from across the entire electromagnetic spectrum has been put to use in industry, research, communication, and medicine.