Radiation is the emission or transmission of energy in a straight line (like a 'ray' in geometry'). This line travels through space or some material, spreading out from the source in all directions; "radiating" out. Radiation can also refer to the emitted energy itself. There are many different types of radiation that can include electromagnetic, thermal, acoustic, particle radiation (such as alpha or beta radiation from a radioactive source), and ionizing radiation.[1]

Ionizing versus Non-Ionizing Radiation

Figure 1. Ionizing radiation is radiation that can strip electrons from atoms. This process is shown above.[2]

Ionizing radiation is a specific type of radiation that has enough energy to eject an electron from some atom. This radiation includes ionizing particles from alpha or beta decay, as well as electromagnetic waves in the form of gamma radiation. Generally speaking, the energies of alpha and beta decay particles and gamma ray photons is higher than the ionization energies of atoms and molecules.[3] These particles ionize the matter and break molecular bonds, which can cause significant biological damage such as burns, radiation sickness, and cancer.

Non-ionizing radiation does not remove electrons from atoms. This means that it is generally less severely damaging than ionizing radiation. Most of the health risks that accompany non-ionizing radiation come from the thermal energy that accompanies the radiation[4]. All forms of radiation can be divided into ionizing and non-ionizing radiation.

Electromagnetic Radiation

Figure 2. The electric (red) and magnetic (blue) fields are changing, causing the radiation moving to the right.
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Electromagnetic radiation is produced by charged particles accelerating through space. As the charge moves, its electric and magnetic fields oscillate like in figure 2.[1] This radiation is also known as an electromagnetic wave as it is composed of alternating electric and magnetic fields. This type of radiation comes in discrete packets known as photons.

There are several different types of electromagnetic radiation, and their properties depend on their energy and wavelength. Some of the different types include radio waves, infrared radiation (felt as heat), microwaves, visible light, ultraviolet radiation, x-rays, gamma rays, and cosmic rays.

Long wavelength electromagnetic radiation (radio to visible light waves) are generally non-ionizing. Shorter wavelength electromagnetic radiation (ultraviolet light to gamma rays) tends to be ionizing (Figure 3).[4]

Figure 3. The electromagnetic spectrum divided into ionizing and non-ionizing radiation.[5]

Thermal Radiation

Thermal radiation is one type of electromagnetic radiation, specifically one that transfers heat often in the form of infrared waves. Generally, thermal radiation and infrared waves are referred to simply as "heat". Since heat is carried by electromagnetic waves, it does not need a physical medium to transfer it. Instead it radiates through space - this is how the Earth is heated by the Sun despite space being a vacuum.[6]

Figure 4. A dog as seen in the infrared spectrum.[7]

Objects emit thermal radiation because they're at a temperature that isn't perfectly cold (absolute zero Kelvin). The thermal energy makes an objects' molecules wiggle. Molecules have electric charge, and that wiggling makes changing electric fields, which makes for radiation. The higher the temperature, the faster the wiggling of the molecules. Objects hotter than ~500°C (773 K) start to emit visible light.[8]

Objects cooler than ~500°C emit radiation that not visible to the naked eye (see Wiens Law for more information]]. Infrared cameras are able to pick up this invisible radiation and digitally convert it into a visible image (such as the one shown in Figure 4). Sometimes this radiant heat is visible. For example, a candle radiates heat. It also gives out visible light, which corresponds with the temperature of the flame. The flame burns hottest at the wick, and gives off blue or white light as it is on the higher end of the visible light spectrum. Surrounding the wick, the flame is yellow and then red in appearance, which is at the lower end of the visible light spectrum. The region surrounding the flame gives off no light as it emits infrared waves, but feels warm to the touch.


Sunlight, also called solar radiation, is a form of radiation that originated from the Sun. The radiation is part of the electromagnetic spectrum, including infrared, visible, and ultraviolet light. Sunlight that is incident upon the Earth's surface has been filtered through the atmosphere, with some of the ultraviolet radiation being absorbed.[9] In addition to providing light for Earth, sunlight also acts as a source of radiant heat, warming the Earth.

For Further Reading


  1. 1.0 1.1 Randall D. Knight. Physics for Scientists and Engineers: A Strategic Approach, 3rd ed.Glenview, IL, U.S.A.: Pearson Education, 2013
  2. Created internally by a member of the Energy Education team.
  3. R. Knight. (August 6, 2015). Physics for Scientists and Engineers, 3rd ed. U.S.A.: Pearson
  4. 4.0 4.1 Centers for Disease Control and Prevention. Radiation Studies, Dec.7, 2015. Accessed on Oct.9, 2018. Available from: https://www.cdc.gov/nceh/radiation/nonionizing_radiation.html
  5. Mirion Technologies. "What is radiation?" Oct 9, 2018. Available from: https://www.mirion.com/introduction-to-radiation-safety/what-is-radiation/
  6. Heat: Heat Transfer. (August 6, 2015). Radiation - Page 1. [Online]. Available: http://www.hk-phy.org/contextual/heat/hea/radia01_e.html.
  7. Wikimedia Commons. (August 6, 2015). Infrared Dog [Online]. Available: http://upload.wikimedia.org/wikipedia/commons/0/0c/Infrared_dog.jpg
  8. The light starts off being red, and then moves through the spectrum as the temperature increases, for more information see: UCSB Science Line, accessed May 10th, 2022. http://scienceline.ucsb.edu/getkey.php?key=6064
  9. ScienceDaily. (May 20, 2015). Sunlight [Online]. Available: http://www.sciencedaily.com/terms/solar_radiation.htm