Gamma rays, electromagnetic wave energy similar to, but of much higher energy than, ordinary X rays. The energy of a quantum is equal to hv ergs, where h is Planck's constant (6.6254 × 10−27 erg sec) and ν is the frequency of the radiation. Gamma rays are emitted by the nuclei of uranium and other radioactive elements. They are highly penetrating, an appreciable fraction being able to traverse several cm of lead. In large amounts they can be harmful to body tissue, but small quantities are used in radiation treatments for cancer.