How can half-life be described in terms of radioactive decay?
It is the time taken for the intensity of the radioactive emission to decay to half of its original intensity.
This is by definition.
Most radioactive decay follows the natural logarithmic law, ie
Lamda is a constant which differs for different radioactive materials and that is why each on has a different half life.
So the half life is the time taken for I to reach exactly half of