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Illustration o the relative abilities o three different types o ionizin radiation tae penetrate solid matter. Alpha pairticles (α) are stopped bi a sheet o paper, while beta particles (β) are stopped bi an aluminium plate. Gamma radiation (γ) is dampened when it penetrates lead.

In pheesics, radiation is a process in which energetic pairticles or energetic waves travel through a vacuum, or through matter-containin media that are nae required for thair propagation. Waves o a mass filled medium itself, such as watter waves or soond waves, are uisually nae considered tae be furms o "radiation" in this sense.