Consider a mixture of a rapidly decaying element A, with a half-life of 1 second, and a slowly decaying element B, with a half-life of 1 year.

Perhaps a puddle of a certain size will evaporate down to half its original volume in one day.

But on the second day, there is no reason to expect that one-quarter of the puddle will remain; in fact, it will probably be much less than that.

As an example, the radioactive decay of carbon-14 is exponential with a half-life of 5,730 years.

A quantity of carbon-14 will decay to half of its original amount (on average) after 5,730 years, regardless of how big or small the original quantity was.

This is an example where the half-life reduces as time goes on.

(In other non-exponential decays, it can increase instead.) The decay of a mixture of two or more materials which each decay exponentially, but with different half-lives, is not exponential.) is the time required for a quantity to reduce to half its initial value.The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo, or how long stable atoms survive, radioactive decay.Rutherford applied the principle of a radioactive element's half-life to studies of age determination of rocks by measuring the decay period of radium to lead-206.Half-life is constant over the lifetime of an exponentially decaying quantity, and it is a characteristic unit for the exponential decay equation.A half-life usually describes the decay of discrete entities, such as radioactive atoms.

Byzantium was already a city eight miles in circumference by the time Herodotus visited it, and it had been liberated from the Persians only thirteen years before.