Radiation is energy that travels in waves or particles through space or a material, such as light, X-rays, or radio waves. It can be useful, like in medicine and communication, but some kinds can be harmful in large amounts.
It comes from Latin “radiatio,” from “radiare” meaning “to shine, emit rays,” and “radius” meaning “ray” or “spoke.” The modern scientific sense developed in the 1800s and 1900s with the study of light, heat, and radioactivity.
Sunlight, Wi‑Fi signals, and X‑rays are all forms of radiation, just with different energies. The scary kind people think of—ionizing radiation—is only a small slice of the spectrum, but it’s powerful enough to damage DNA, which is why it can both treat cancer and cause it.
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