Ionising radiation carries enough energy to remove electrons from atoms and molecules. In living cells, that can damage important molecules including DNA. Uncontrolled DNA damage can increase the risk of cancer. But the same ability to damage cells can be used deliberately in radiotherapy to destroy cancer cells or stop them dividing. Doctors shape and aim radiation beams so the tumour receives a high dose while healthy tissue receives as little as practical. Treatment is carefully planned because healthy cells can also be harmed. The effect depends on dose, type of radiation and where the energy is deposited, which is why radiation can be both a hazard and a medical tool.

The useful part of this physics is that it helps people predict and design real things. Engineers often begin with the same simplified model used in school, test where it works, then add more detail when measurements show that the real situation is more complicated.

Radioactive decay happens in unstable atomic nuclei and is random for any individual nucleus, although large samples follow predictable patterns. Alpha, beta and gamma radiation have different penetrating and ionising abilities, so their usefulness and risk depend on both type and exposure. Ionisation can damage cells, but carefully controlled radiation can also be aimed at unwanted tissue or used as a tracer because it is detectable in very small quantities.