Radioactivity is the emission of energetic particles or waves from atoms. Natural radiation occurs when unstable nuclei transform to some other nucleus by emitting radiation. Induced radiation often occurs when electrons which have been excited lose energy in the form of X-rays or microwaves, as done in dentist offices, microwave ovens, and radar. As it applies to nuclear energy, many materials created during the operation of a reactor are unstable. As they decay over varying lengths of time (from microseconds to hundreds of thousands of years), they emit energetic particles or waves.