Half life is the time period it takes for a substance to decay to one half its original value. In other words, if we start with (say) 100 g of a substance and its half life is 1 day, then after 1 day, we will be left with only 50 g of the substance.
Half life can be determined by calculating the amount of a substance at any two given time intervals. A number of techniques are available to determine the amount or quantity of a given substance. These could be as simple as weighing them or can be very complex. Let the time duration is t unit, over which the substance's quantity changed from M1 to M2, then the half life (n unit) can be calculated as:
`(1/2)^(t/n) = (M2)/(M1)`
For example, if in 10 years, the amount of a substance decreased from 100 g to 25 g, then its half life is
`(1/2)^(10/n) = 25/100`
solving this, we get n = 5 years.
Hope this helps.