|Three types of photons|
The energy of a photon varies inversely with its wavelength. Photons are usually emitted by molecules, with energies of a few electronvolts and wavelengths of a fraction of a micron (a thousandth of millimetre). X-rays are emitted by the deeper layers of the atom, with energies reaching hundreds of thousands of electronvolts (keV) and wavelengths of the order of a billionth of a metre (nanometres). Gamma rays, on the other hand, are emitted by nuclei and have even higher energies with wavelengths appropriate to the dimensions of a nucleus.