Radiation: Definition, Types & Examples

Radiation might have gotten a bad rap from nuclear accidents, but the word "radiation" actually encompasses a large range of phenomena. Radiation is everywhere, and a great number of everyday electronic devices rely on it. Without radiation from the sun, life on Earth would look very different, if it existed at all.

The basic definition of radiation is simply the emission of energy, in the form of photons or other subatomic particles. Whether or not radiation is dangerous depends on how much energy those particles have. The types of radiation are distinguished by the types of particles involved and their energies.

Electromagnetic Radiation

Electromagnetic radiation is emitted energy in the form of waves called electromagnetic waves, or light. According to quantum mechanics, light is both a particle and a wave. When it is being considered as a particle, it is called a photon. When it is being considered as a wave, it is called an electromagnetic wave or a light wave.

Light is classified depending on its wavelength, which is inversely proportional to its energy: Long wavelength light has lower energy compared to short wavelength light. Its wavelength spectrum is most commonly divided into: radio waves, microwaves, infrared, visible light, ultraviolet radiation, x-rays and gamma rays. When light is emitted as electromagnetic radiation, this radiation is classified by these categories as well.

Electromagnetic radiation (which, to re-emphasize, is just light) is ubiquitous in the universe and here on earth. Lightbulbs radiate visible light; microwaves radiate microwaves. A remote control radiates infrared to send a signal to a television. These types of radiation are low-energy and are generally not harmful in the amounts that humans are normally exposed to.

The part of the spectrum with shorter wavelengths than visible light can do damage to human tissue. Ultraviolet light, right next to visible light on the spectrum, can cause sunburns and skin cancer.

Radiation from the higher-energy end of the ultraviolet spectrum, in addition to X-rays and gamma rays, is known as ionizing radiation: It is energetic enough to be able to knock electrons off of atoms, turning the atoms into ions. Ionizing radiation can damage DNA and cause a multitude of health problems.

Radiation From Space

The radiation from stars, supernovae and black hole jets is what allows astronomers to see them. Gamma ray bursts, for example, are very energetic explosions that are the brightest radiation events known to occur in the universe. The radiation detected from faraway suns allows astronomers to deduce their age, size and type.

Space is also full of cosmic rays: Fast-moving protons and atomic nuclei that zip through the cosmos at nearly the speed of light that are much, much heavier than photons. Because of their mass and speed, they have incredibly high amounts of energy.

On earth, the danger posed by cosmic rays is negligible. The energy of these particles is mostly spent breaking up chemical bonds in the atmosphere. However, cosmic rays are a major consideration for humans in space.

Trips in low-Earth orbit, including the International Space Station, are still protected from cosmic rays by several factors. However, any long-term crewed mission beyond low-Earth orbit, to Mars, for example, or to the Moon for an extended mission, has to mitigate the health dangers of cosmic rays to its astronauts.

Radioactive Decay

The nuclei of a radioactive substance or radioactive material, such as uranium or radon, are unstable. To stabilize, the nuclei will undergo nuclear reactions, including spontaneously breaking apart, letting off energy when they do. This energy is emitted in the form of particles. The particles emitted when the substance decays determine what type of decay it is. There are three main types of radiation from nuclear decay: alpha radiation, beta radiation and gamma radiation.

Gamma radiation is the simplest, as it is a high-energy photon emitted from the radioactive atom with a wavelength in the gamma part of the spectrum.

Beta radiation is the transmutation of a proton into a neutron, facilitated by the emission of an electron. This process can also happen in reverse (transforming a neutron into a proton) by emitting a positron, which is the positively-charged antimatter counterpart of an electron. These particles are referred to as beta particles despite also having other names.

Alpha radiation is the emission of an "alpha particle," which is made of two neutrons and two protons. This is also a standard helium nucleus. After this decay, the original atom has its atomic number decreased by 2, changing its elemental identity, and its atomic weight decreased by 4. All three kinds of decay radiation are ionizing.

Radioactive decay has many uses, including radiation therapy, radiocarbon dating, and so on.

Radiative Heat Transfer

Heat energy can be transferred from one location to another via electromagnetic radiation. This is how heat reaches the Earth through the vacuum of space from the Sun.

The color of an object affects how well it can absorb heat. White reflects most wavelengths, while black absorbs. Silver and shiny objects also reflect. The more reflective something is, the less radiative energy it will absorb, and the less it will heat up when exposed to radiation. This is why black objects become hotter in the sun than white objects.

Good light absorbers, such as black objects, are also good emitters when they are warmer than their surroundings.

The Greenhouse Effect

If radiation passes through a transparent or semi-transparent material into an enclosed region, it can become trapped when it is absorbed and re-emitted at different wavelengths.

This is why your car gets so hot in the sun even if it is only 70 outside; the surfaces inside your car absorb the radiation from the sun, but re-emit it as heat at wavelengths that are too long to penetrate the window glass. So, instead, the heat energy stays trapped within the car.

This also happens with Earth's atmosphere. Sun-warmed earth and ocean will re-emit some absorbed heat at different wavelengths than the sunlight originally had. This will make it impossible for the heat to return through the atmosphere, keeping it trapped closer to the Earth.

Blackbody Radiation

A blackbody is a theoretical, ideal object that absorbs all wavelengths of light and emits all wavelengths of light. However, it emits light of different wavelengths at different intensities.

The intensity of the light, or flux, can be described as the number of photons per unit area being emitted from the black body. A blackbody spectrum, with wavelength on the x-axis and flux on the y-axis, will always show a peak at a certain wavelength; more photons are emitted with this energy than any other value of energy.

This peak changes depending on the temperature of the blackbody according to Wien's Displacement Law: The peak will decrease linearly in wavelength as the temperature of the blackbody increases.

Knowing this relation, astronomers often model stars as perfect blackbodies. While this is an approximation, it gives them a good estimate for the temperature of the star, which can tell them about where it is in its lifecycle.

Another important blackbody relation is the Stefan-Boltzmann Law, which says that the total energy radiated by a blackbody is proportional to its temperature taken to the fourth power: E ∝ T4.

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About the Author

Meredith is a science writer and physicist based in Seattle. She received her Bachelor of Science degree in physics from the University of Illinois at Urbana-Champaign and her Master of Science degree in physics from the University of Washington. She has written for Live Science, Physics, Symmetry, and WIRED, and was an AAAS Mass Media Fellow in 2019.