Transcript: The physical process of radioactivity was discovered accidentally in the late nineteenth century by French physicist Antoine Becquerel. He had left some uranium bearing minerals near a photographic plate in a drawer in his laboratory and came back to find the plate fogged, indicating that particles or emissions had gone from the radioactive material to the photographic plate. Radioactivity involves the decay spontaneously of an element to another form, either a form of the same element with a different number of neutrons, which is an isotope, or an actual change of the number of protons, in which case it’s a new element. The parent isotope is the starting point, and the daughter isotope is the end point. The point about radioactivity that lets people calculate ages with it is the time taken for one half of the original atoms to decay to the daughter form is a fixed number determined purely by atomic processes; it's called the half-life. The half-life of a radioactive decay process does not depend on any other physical condition, and so if we can measure the starting composition of parent and daughter molecules or atoms and the ending composition we can infer how long has past in the decay process. This is the way that radioactivity is used to measure ages.