Which of the following is an example of non-ionizing radiation?

• A. X-rays
• B. Gamma rays
• C. Ultraviolet light
• D. Microwave radiation

Answer: (C)

The main idea is that ionizing radiation has enough photon energy to eject electrons from atoms, while non-ionizing radiation does not. X-rays and gamma rays have very high-energy photons, so they readily ionize atoms and molecules. Microwave photons are low-energy and mainly cause heating or molecular rotation, not ionization. Ultraviolet light, in many practical biosystems contexts, is treated as non-ionizing because its photons are not typically energetic enough to ionize atoms in biological tissues under ordinary exposure; instead, they can cause photochemical effects and heating. So ultraviolet light is the correct example of non-ionizing radiation in this scenario.