Essentially indistinguishable from gamma rays are the X-rays, highly energetic electromagnetic (E&M) photons. The wavelengths of these photons range from about 10^-8m to 10^-12m, with corresponding energies in the range from about 100 eV to several hundred keVs.

The first discoverer of X-rays was Wilhelm Röentgen, a German physicist who, in 1895, accidentally found these "light" rays when he put a radioactive source in a drawer with some unexposed photographic negatives and found the next day that the film had been exposed somehow! The radioactive source had emitted X-rays and produced bright spots on the film. The era of the X-ray and the rest of the electromagnetic spectrum had begun.

X-rays are often used in medicine to see inside the body, without invasive surgery. The easiest method is to simply use photographic paper behind the target (the body) and shine X-rays through the target onto the film. Because the bones are much denser than the rest of the tissue, the X-rays don't get through them as well, so the bones show up as a dark areas on the negative film. The rest of the tissue is relatively transparent to the X-rays and show up as light areas on the film.

Unfortunately, people at first did not understand the power of the X-rays in penetrating body tissue or just about anything else. The first types of X-rays used were often the most energetic, which easily pass through human tissue, and can alter the genetic information of cells, often making them cancerous in the process. Nowadays, safer low-energy X-rays are used in hospitals and dentists' offices, though lead sheilding is still recommended for indirect exoposure.