# Radiation

Radiation is energy emitted by a source that moves through matter or empty space, and examples of it include light, heat, and sound.<sup>[1](https://www.energy.gov/sites/prod/files/2013/03/f0/QA%20Book.pdf)</sup> In everyday and technical use, the word most often refers to the emissions from radioactive atoms and from devices such as X-ray machines, which are classified as ionizing radiation because they can produce charged particles (ions) in matter<sup>[1](https://www.energy.gov/sites/prod/files/2013/03/f0/QA%20Book.pdf)</sup>. Radiation takes two broad forms: electromagnetic, carried by waves and their associated photons, and particulate, in which mass is given off with the energy of motion<sup>[1](https://www.energy.gov/sites/prod/files/2013/03/f0/QA%20Book.pdf)</sup><sup> • </sup><sup>[2](https://www.nist.gov/glossary-term/46521)</sup>.

| Key fact | Detail |
|---|---|
| Definition | Energy emitted from a source that travels through material or space; light, heat, and sound are types of radiation<sup>[1](https://www.energy.gov/sites/prod/files/2013/03/f0/QA%20Book.pdf)</sup> |
| Main forms | Electromagnetic (gamma rays, X-rays, light, radio) and particulate (alpha and beta particles, neutrons)<sup>[1](https://www.energy.gov/sites/prod/files/2013/03/f0/QA%20Book.pdf)</sup><sup> • </sup><sup>[3](https://www.energy.gov/sites/prod/files/2013/07/f2/DOE-HDBK-1130-2008-CN1.pdf)</sup> |
| Ionizing vs non-ionizing | Radiation with enough energy to knock electrons off atoms or molecules is ionizing; EM forms with frequency under 3 × 10<sup>15</sup> Hz fall in the non-ionizing range<sup>[4](https://www.energy.gov/documents/finding-no-significant-impact-cleanup-and-closure-energy-engineering-and-technology)</sup><sup> • </sup><sup>[5](https://www.energy.gov/sites/prod/files/2016/04/f30/NuclearPoster_press_09Jan2014_0.pdf)</sup> |
| Four basic ionizing types | Alpha particles, beta particles, gamma or X rays, and neutrons<sup>[3](https://www.energy.gov/sites/prod/files/2013/07/f2/DOE-HDBK-1130-2008-CN1.pdf)</sup> |
| Alpha particle | Two protons and two neutrons, charge +2, range about 1–2 inches in air, stopped by a sheet of paper or the dead layer of skin<sup>[3](https://www.energy.gov/sites/prod/files/2013/07/f2/DOE-HDBK-1130-2008-CN1.pdf)</sup> |
| Wave relations | Wavelength and frequency are linked by \( c = \lambda f \), and photon energy by Planck's formula \( E = hf \)<sup>[6](https://openstax.org/books/astronomy/pages/5-summary)</sup> |
| Natural background | Background radiation comes from cosmic rays and naturally occurring radioactive materials; the largest source of ionizing radiation for the average American is naturally occurring radon in the air<sup>[1](https://www.energy.gov/sites/prod/files/2013/03/f0/QA%20Book.pdf)</sup><sup> • </sup><sup>[7](https://www.energy.gov/science/doe-explainsradioactivity)</sup> |

## Electromagnetic radiation

[Electromagnetic radiation](https://www.edgepedia.org/electromagnetic-radiation) is the emission or transfer of energy in the form of electromagnetic waves with the associated photons<sup>[2](https://www.nist.gov/glossary-term/46521)</sup>. It is generated by a moving electric charge: an oscillating charge produces an electric field and a magnetic field that are perpendicular to each other, and the resulting wave propagates away from the source<sup>[8](https://openstax.org/books/physics/pages/15-1-the-electromagnetic-spectrum)</sup>. [James Clerk Maxwell](https://www.edgepedia.org/james-clerk-maxwell) showed that charged particles changing their motion, as they do in every atom and molecule, give off such waves<sup>[6](https://openstax.org/books/astronomy/pages/5-summary)</sup>.

All electromagnetic waves travel at the speed of light in a vacuum, and their energy is set not by that speed but by their frequency<sup>[5](https://www.energy.gov/sites/prod/files/2016/04/f30/NuclearPoster_press_09Jan2014_0.pdf)</sup>. Wavelength \( \lambda \), frequency \( f \), and the speed of light \( c \) are related by \( c = \lambda f \), so wavelength decreases as frequency increases<sup>[6](https://openstax.org/books/astronomy/pages/5-summary)</sup>. Electromagnetic radiation also behaves in some experiments like a wave and in others like a stream of particles, each packet of energy being a photon whose energy follows Planck's formula \( E = hf \)<sup>[6](https://openstax.org/books/astronomy/pages/5-summary)</sup>.

**The spectrum.** The electromagnetic spectrum runs from radio waves through microwaves, infrared, visible light, ultraviolet, and X-rays to gamma rays<sup>[6](https://openstax.org/books/astronomy/pages/5-summary)</sup>. Visible light is a narrow band whose frequencies run from about \( 4.0 \times 10^{14} \ \mathrm{s^{-1}} \) at the red end to \( 7.9 \times 10^{14} \ \mathrm{s^{-1}} \) at the violet end<sup>[8](https://openstax.org/books/physics/pages/15-1-the-electromagnetic-spectrum)</sup>. [Gamma rays](https://www.edgepedia.org/gamma-rays), at the high-frequency end, are electromagnetic radiation with wavelengths no longer than 0.01 nanometer<sup>[9](https://openstax.org/books/astronomy-2e/pages/5-2-the-electromagnetic-spectrum)</sup>. Each band has distinct uses: radio and TV for communications, microwaves for communications, radar, and cooking, infrared for thermal imaging, ultraviolet for sterilization, and X-rays and gamma rays for medical diagnosis and cancer therapy<sup>[8](https://openstax.org/books/physics/pages/15-1-the-electromagnetic-spectrum)</sup>.

## Ionizing and non-ionizing radiation

[Ionizing radiation](https://www.edgepedia.org/ionizing-radiation) is radiation with sufficient energy to displace electrons from atoms or molecules, creating ions; it can be electromagnetic, such as X-rays or gamma radiation, or subatomic particles, such as alpha and beta radiation<sup>[4](https://www.energy.gov/documents/finding-no-significant-impact-cleanup-and-closure-energy-engineering-and-technology)</sup>. The ions can interact with other atoms and molecules, and in biological systems this interaction can cause damage to tissue or the organism<sup>[4](https://www.energy.gov/documents/finding-no-significant-impact-cleanup-and-closure-energy-engineering-and-technology)</sup>. Electromagnetic radiation that lacks enough energy to ionize an atom is called non-ionizing radiation; examples are radar waves, microwaves, and visible light<sup>[3](https://www.energy.gov/sites/prod/files/2013/07/f2/DOE-HDBK-1130-2008-CN1.pdf)</sup>. On the electromagnetic spectrum, forms with a frequency under \( 3 \times 10^{15} \) Hz fall into the non-ionizing range<sup>[5](https://www.energy.gov/sites/prod/files/2016/04/f30/NuclearPoster_press_09Jan2014_0.pdf)</sup>.

Ionizing radiation is produced both by unstable atoms and by devices such as X-ray machines, accelerators, and fluoroscopes<sup>[1](https://www.energy.gov/sites/prod/files/2013/03/f0/QA%20Book.pdf)</sup><sup> • </sup><sup>[3](https://www.energy.gov/sites/prod/files/2013/07/f2/DOE-HDBK-1130-2008-CN1.pdf)</sup>.

## Radioactive decay and its emissions

Radioactivity is the release of energy from the decay of the nuclei of certain kinds of atoms and isotopes<sup>[7](https://www.energy.gov/science/doe-explainsradioactivity)</sup>. Unstable atoms have an excess of energy or mass, or both, and to reach stability they emit the excess in the form of particles or rays<sup>[1](https://www.energy.gov/sites/prod/files/2013/03/f0/QA%20Book.pdf)</sup>. The result, radioactive decay, transforms an unstable atom (a radionuclide) into a different atom as it reaches a more stable, lower-energy configuration<sup>[4](https://www.energy.gov/documents/finding-no-significant-impact-cleanup-and-closure-energy-engineering-and-technology)</sup>.

[Radioactive decay](https://www.edgepedia.org/radioactive-decay) produces three main types of ionizing radiation, alpha particles, beta particles, and gamma or X-rays, which the senses cannot detect<sup>[4](https://www.energy.gov/documents/finding-no-significant-impact-cleanup-and-closure-energy-engineering-and-technology)</sup>. The four basic types of ionizing radiation of concern in the DOE complex add neutrons to this list<sup>[3](https://www.energy.gov/sites/prod/files/2013/07/f2/DOE-HDBK-1130-2008-CN1.pdf)</sup>. Their properties differ sharply:

- **Alpha particles** have a large mass, consisting of two protons, two neutrons, and no electrons, and carry a charge of plus two<sup>[3](https://www.energy.gov/sites/prod/files/2013/07/f2/DOE-HDBK-1130-2008-CN1.pdf)</sup>. They deposit a large amount of energy over a short distance, have a range in air of about 1–2 inches, and are stopped by a few centimeters of air, a sheet of paper, or the dead outer layer of skin<sup>[3](https://www.energy.gov/sites/prod/files/2013/07/f2/DOE-HDBK-1130-2008-CN1.pdf)</sup>. They are therefore not an external hazard, but if inhaled or ingested they come into direct contact with tissue cells and can cause severe damage, making them a serious internal hazard<sup>[3](https://www.energy.gov/sites/prod/files/2013/07/f2/DOE-HDBK-1130-2008-CN1.pdf)</sup><sup> • </sup><sup>[10](https://ehss.energy.gov/OHRE/roadmap/experiments/0491terms.html)</sup>.
- **Beta particles** are electrons emitted in decay; alpha and beta particles are not part of the electromagnetic spectrum and travel slower than the speed of light<sup>[5](https://www.energy.gov/sites/prod/files/2016/04/f30/NuclearPoster_press_09Jan2014_0.pdf)</sup>.
- **Gamma rays and X-rays** are electromagnetic, so they penetrate farther than alpha or beta particles; gamma-ray energy is deposited across the body, in contrast to the local energy deposition of an alpha particle<sup>[1](https://www.energy.gov/sites/prod/files/2013/03/f0/QA%20Book.pdf)</sup><sup> • </sup><sup>[4](https://www.energy.gov/documents/finding-no-significant-impact-cleanup-and-closure-energy-engineering-and-technology)</sup>.

Because each type has different characteristics, each requires different amounts of material to shield it<sup>[4](https://www.energy.gov/documents/finding-no-significant-impact-cleanup-and-closure-energy-engineering-and-technology)</sup>.

## Natural background and exposure

There is a background of natural radiation in the environment, coming from cosmic rays and from naturally occurring radioactive materials<sup>[1](https://www.energy.gov/sites/prod/files/2013/03/f0/QA%20Book.pdf)</sup>. Radioactivity has always been present on Earth, though it was not scientifically studied until 1896, when [Henri Becquerel](https://www.edgepedia.org/henri-becquerel), Pierre Curie, and [Marie Curie](https://www.edgepedia.org/marie-curie) began its investigation<sup>[7](https://www.energy.gov/science/doe-explainsradioactivity)</sup>. For the average American, the largest source of ionizing radiation is naturally occurring radon in the air<sup>[7](https://www.energy.gov/science/doe-explainsradioactivity)</sup>.

Exposure to penetrating radiation is called irradiation, and it can come from sources used in medicine and industry as well as natural ones<sup>[1](https://www.energy.gov/sites/prod/files/2013/03/f0/QA%20Book.pdf)</sup>. A useful distinction in radiation protection is that radiation is a type of energy, while contamination is radioactive material that is uncontained and in an unwanted place; exposure to ionizing radiation without exposure to radioactive material does not result in contamination<sup>[3](https://www.energy.gov/sites/prod/files/2013/07/f2/DOE-HDBK-1130-2008-CN1.pdf)</sup>.

## Measurement

[Absorbed dose](https://www.edgepedia.org/absorbed-dose) is measured as the amount of energy deposited per unit mass of tissue, a quantity expressed in rads<sup>[10](https://ehss.energy.gov/OHRE/roadmap/experiments/0491terms.html)</sup>. Because different types of ionizing radiation differ in biological effectiveness, effective dose is expressed in rem (roentgen equivalent in man), a unit corrected for the varying biological effectiveness of the various radiation types<sup>[11](https://ehss.energy.gov/ohre/roadmap/achre/glossary.html)</sup>.

## Uses

Radioactivity is put to work by scientists and engineers in several ways: it heats satellites, enables medical imaging and cancer treatments aimed at tumors, allows radiometric dating, and aids studies of nature's laws and how matter originated.<sup>[7](https://www.energy.gov/science/doe-explainsradioactivity)</sup> Radioactive isotopes also serve in medicine, chemistry, energy, environmental sciences, material sciences, manufacturing, and national security<sup>[7](https://www.energy.gov/science/doe-explainsradioactivity)</sup>. On the non-ionizing side, everyday technologies rely on low-frequency radiation: radio and television waves, cell phones, electric lighting, radar, and infrared remote controls<sup>[5](https://www.energy.gov/sites/prod/files/2016/04/f30/NuclearPoster_press_09Jan2014_0.pdf)</sup>. Kitchen microwave ovens, first marketed as Radaranges because they were based on World War II radar technology, use a frequency of \( 2.45 \times 10^{9} \) Hz to make polar molecules such as water rotate faster, releasing the rotational energy as heat<sup>[8](https://openstax.org/books/physics/pages/15-1-the-electromagnetic-spectrum)</sup>.

## References

1. [What is radiation? (Radiation Answers booklet), U.S. Department of Energy](https://www.energy.gov/sites/prod/files/2013/03/f0/QA%20Book.pdf)
2. [Electromagnetic radiation, NIST/OSAC Lexicon](https://www.nist.gov/glossary-term/46521)
3. [DOE-HDBK-1130-2008, Radiological Worker Training, Module 1: Radiological Fundamentals](https://www.energy.gov/sites/prod/files/2013/07/f2/DOE-HDBK-1130-2008-CN1.pdf)
4. [Finding of No Significant Impact, Cleanup and Closure, U.S. Department of Energy](https://www.energy.gov/documents/finding-no-significant-impact-cleanup-and-closure-energy-engineering-and-technology)
5. [Nuclear Energy Poster, U.S. Department of Energy](https://www.energy.gov/sites/prod/files/2016/04/f30/NuclearPoster_press_09Jan2014_0.pdf)
6. [Ch. 5 Summary, Astronomy, OpenStax](https://openstax.org/books/astronomy/pages/5-summary)
7. [DOE Explains... Radioactivity, U.S. Department of Energy Office of Science](https://www.energy.gov/science/doe-explainsradioactivity)
8. [15.1 The Electromagnetic Spectrum, Physics, OpenStax](https://openstax.org/books/physics/pages/15-1-the-electromagnetic-spectrum)
9. [5.2 The Electromagnetic Spectrum, Astronomy 2e, OpenStax](https://openstax.org/books/astronomy-2e/pages/5-2-the-electromagnetic-spectrum)
10. [Radiation Terms, Roadmap to the Project, U.S. Department of Energy](https://ehss.energy.gov/OHRE/roadmap/experiments/0491terms.html)
11. [Glossary, Advisory Committee on Human Radiation Experiments, U.S. Department of Energy](https://ehss.energy.gov/ohre/roadmap/achre/glossary.html)

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*Topic: Encyclopedia › Physical world and mathematics › Physics › Matter and radiation physics › Quantum optics and photonics › Nonclassical light and photon statistics*

*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 15, 2026 · Edited: Sep 30, 2026; Oct 15, 2026 · Last review: Oct 15, 2026*

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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license