Electromagnetic radiation; 10. X-rays
These have a wavelength of 0.01 to 10 nm, hence photon energies of 1keV to 1000keV. They were discovered by Wilhelm Roentgen in 1895.
Often, X-rays are divided into hard X-rays (wavelength of 0.01 to 0.1 nm, photon energies of 10keV to 1000keV) and soft X-rays (wavelength of 0.1 to 10 nm, photon energies of 0.1keV to 10keV). In general, shorter wavelength X-rays are said to be harder than longer wavelength ones.
X-rays are widely used in medicine and technology because they can penetrate many materials hence reveal internal structures. Excessive exposure of living beings to X-rays, especially very hard ones, can be damaging and even fatal. Long-term exposure to even moderate levels can cause cancer in humans.