The German physicist Wilhelm Röntgen, also studying cathode rays, made a remarkable discovery in 1895. He had sealed a cathode and a metal target electrode into a vacuum tube, as shown in FIGURE 37.10, and then applied a much higher voltage than normally used to produce cathode rays. He happened, by chance, to leave a sealed envelope containing photographic film near the vacuum tube, and was later surprised to discover that the film had been exposed. This serendipitous discovery was the beginning of the study of x rays.

FIGURE 37.10 Röntgen’s x-ray tube.

Röntgen quickly found that the vacuum tube was the source of whatever was exposing the film. Not having any idea what was coming from the tube, he called them x rays, using the algebraic symbol x as meaning “unknown.” X rays were unlike anything, particle or wave, ever discovered. Röntgen was not successful at reflecting the rays or at focusing the rays with a lens. He showed that they travel in straight lines, like particles, but they also pass right through most solid materials, something no known particle could do.

Scientists soon began to suspect that x rays were an electromagnetic wave with a wavelength much shorter than that of visible light. However, it wasn’t until 20 years after their discovery that this was verified by the diffraction of x rays, showing that they have wavelengths in the range 0.01 nm to 10 nm. At the time, the properties of x rays seemed far outside the scope of Maxwell’s theory of electromagnetic waves.