radiation

upright=1.5|thumb|right|An illustration of the relative abilities of three different types of ionizing radiation to penetrate solid matter. Typical alpha particles (α) are stopped by a sheet of paper, while beta particles (β) are stopped by 3mm aluminum foil. Gamma radiation (γ) is dampened when it penetrates lead. Note caveats in the text about this simplified diagram. border|right|frameless|597x597px thumb|upright|The international symbol for ionizing radiation (radioactivity) that is unsafe for Radiation shield|unshielded humans. Radiation, in general, exists throughout nature, such as in light and sound.

In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or a material medium. This includes: electromagnetic radiation consisting of photons, such as radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, and gamma radiation (γ) particle radiation consisting of particles of non-zero rest energy, such as alpha radiation (α), beta radiation (β), proton radiation and neutron radiation acoustic radiation, such as ultrasound, sound, and seismic waves, all dependent on a physical transmission medium gravitational radiation, in the form of gravitational waves, ripples in spacetime