Editing Electromagnetic spectrum (section)

=== Gamma rays ===
{{Main|Gamma rays}}
After hard X-rays come [[gamma rays]], which were discovered by [[Paul Ulrich Villard]] in 1900. These are the most energetic [[photons]], having no defined lower limit to their wavelength. In [[astronomy]] they are valuable for studying high-energy objects or regions, however as with X-rays this can only be done with telescopes outside the Earth's atmosphere. Gamma rays are used experimentally by physicists for their penetrating ability and are produced by a number of  [[radioisotopes]]. They are used for [[irradiation]] of foods and seeds for sterilization, and in medicine they are occasionally used in [[Radiation oncology|radiation cancer therapy]].<ref>[http://www.revisionworld.com/gcse-revision/physics/waves/uses-electromagnetic-waves Uses of Electromagnetic Waves | gcse-revision, physics, waves, uses-electromagnetic-waves | Revision World<!-- Bot generated title -->]</ref> More commonly, gamma rays are used for diagnostic imaging in [[nuclear medicine]], an example being [[Positron emission tomography|PET scans]]. The wavelength of gamma rays can be measured with high accuracy through the effects of [[Compton scattering]].