Shining visible radiation from a torch beam onto a hand lights the hand up because the hand has been exposed to light.
Irradiation from radioactive decay can damage living cells. This can be put to good use as well as being a hazard.
Irradiation can be used to preserve fruit sold in supermarkets by exposing the fruit to a radioactive source - typically cobalt-60. The gamma rays emitted by the cobalt will destroy any bacteria on the fruit but will not change the fruit in any significant way. The process of irradiation does not cause the irradiated object to become radioactive.
Doctors also use radioactive sources for a number of reasons, eg:
These beams are aimed at the tumour from many different directions to maximise the dose on the tumour but to minimise the dose on the surrounding soft tissue. This technique can damage healthy tissue, so careful calculations are done to establish the best dose - enough to kill the tumour, but not so much so that the healthy tissue is damaged.
In medical applications that involve using radioactive sources, efforts are made to ensure that irradiation does not cause any long-term effects. This is done by considering:
If the half-life chosen is too long, the damaging effects of the radiation would last for too long and the dose received would continue to rise.