Invisible radiation
Visible light has been known since organisms developed eyes. Today we pluck long waves of invisible radiation from our surroundings with cell phones, television receivers, and radios, while we wear sunglasses and sunscreen in part to protect against shorter waves that we cannot see. The perception that invisible radiation pervades the universe, however, is relatively new. In 1800 William Herschel discovered that invisible rays, now called infrared radiation, heat a thermometer better than visible sunlight. The following year Johann Bitter found the other side of the story: Invisible ultraviolet radiation affects silver chloride more than light does.
Like most discoveries of invisible rays, the discoveries of Herschel and Bitter were accidental. The next discovery however, was predicted. In 1873 James Clerk Maxwell incorporated infrared and ultraviolet radiation along with visible light into his theory of electromagnetic waves. Maxwell's theory forecast that radiation with wavelengths longer than infrared and shorter than ultraviolet should exist. In 1888 Heinrich Hertz was able to use Maxwell's theory to produce and detect the invisible long waves, which we now know as radio waves. But no one knew how to make the very short waves.
In the 19th century, however, one different form of invisible radiation was known. William Crookes, while trying to find whether electricity would pass through a vacuum, had discovered in 1878 that an invisible ray initiated by electric current could be detected by the fluorescence it produced where it struck. These rays, which proceeded from an electric terminal called the cathode, came to be known as cathode rays. In 1897 they were identified as streams of tiny charged particles (electrons), quite different from the electromagnetic waves analyzed by Maxwell.
Cathode rays were to lead to the next two accidents that uncovered new forms of radiation, which in turn started what some have called the second scientific revolution. In 1895 Wilhelm Roentgen was investigating materials that fluoresce when exposed to cathode rays. He had wrapped his tube in black paper, known to stop cathode rays, and was going to view a sheet coated with a fluorescent material in the dark. But he noticed that the sheet started to fluoresce even before he removed the black paper from his Crookes tube. Since Crookes had established that cathode rays do not penetrate such a shield, something else had to be causing the fluorescence. Roentgen took his fluorescent material into a closet, where it continued to glow whenever the Crookes tube was turned on. A series of experiments showed that this invisible radiation could pass through metals as well as through layers of paper, so it was not the same as the cathode rays. Roentgen had discovered X rays, the very short waves predicted by Maxwell's theory, although he initially failed to recognize them as short electromagnetic radiation. Within days of his announcement of these rays, doctors began to use them to aid in seeing inside the human body.
Within three months of Roentgen's announcement of X rays, another form of invisible radiation was found. Henri Becquerel thought that there might be X rays produced by fluorescent materials. He was working with a uranium compound that became fluorescent when exposed to sunlight. When he exposed a photographic plate that was wrapped in black paper to the fluorescing uranium compound, he got a faint image on the plate. Luck brought Becquerel a series of cloudy days, so he could not expose the compound to sunlight. Becquerel left the sample and the wrapped plate in a drawer. When the cloudy weather persisted, he decided to develop the photograph anyway, hoping for a slight exposure. Instead, he found that the plate was heavily fogged. Radiation had been emitted by the uranium compound continuously, without the stimulation of sunlight.
Becquerel and other physicists soon found that the invisible radiation given off by the uranium compound -- which in 1898 Marie Curie named radioactivity -- consisted of three parts, alpha, beta, and gamma rays. Becquerel himself was the first to show that beta rays are the same as cathode rays; that is, streams of electrons. In 1900 Ernest Rutherford was able to show that gamma rays are basically the same as X rays, but with even shorter wavelengths. A few years later, Rutherford and Hans Geiger found that alpha rays are helium atoms that have been stripped of their electrons. In a sense, then, Becquerel's radioactivity was not a new type of radiation. Instead, its significance was that it was radiation spontaneously emitted by atoms. From that discovery, many others have flowed.