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 an aluminum plate. Gamma radiation (γ) is damped when it penetrates lead. Note caveats in the text about this simplified diagram.
The international symbol for types and levels of radiation that are unsafe for unshielded humans. Radiation in general exists throughout nature, such as in light and sound.