Half Life - GCSE Physics Definition

What is Half Life?

In GCSE Physics, the half-life of a radioactive sample is the time it takes for the number of nuclei in the sample to decrease by half. For a slightly different, but equivalent wording: half-life is the time taken for half the undecayed nuclei to decay. An alternative definition is that half-life is the time taken for the activity of a source to decay by half.

The exact definition you need to learn depends on which of the GCSE Physics exam boards you will take your exam with. With each half-life that passes, the amount of undecayed nuclei remaining halves. This can be represented using a graph of activity against time. This type of graph also makes it easier to calculate the half-life of a sample.

Radioactive decay is spontaneous and random, so it is impossible to know when a particular unstable nucleus will decay. The half-life of an isotope is the rate at which the activity of a sample decreases. Different isotopes have different half-lives and half-lives can vary from a fraction of a second to billions of years in length. An isotope with a short half-life contains nuclei that decay very quickly. The most unstable nuclei have the shortest half-lives.