Back More
Salem Press

Table of Contents

Principles of Physics

Radiation

by Kenrick Vezina, MS

Fields of Study

Electromagnetism; Atomic Physics; Nuclear Physics

Summary

Radiation is energy moving as electromagnetic waves or subatomic particles. Most radiation falls within the electromagnetic spectrum. Electromagnetic radiation is divided into categories based on wavelength. Some common activities associated with radiation include receiving music translated into radio waves or applying sunscreen to prevent skin damage from ultraviolet radiation.

Principal Terms

  • alpha radiation: alpha (α) particles typically emitted during alpha decay, a subtype of radioactive decay.

  • beta radiation: beta (β) rays emitted during beta decay, a subtype of radioactive decay.

  • gamma radiation: electromagnetic radiation with a wavelength shorter than 1 × 10−11 meters; gamma (γ) rays typically emitted during gamma decay, a subtype of radioactive decay.

  • inverse-square law: radiation emanating from a single point has an intensity inversely proportional to the square of the distance the radiation has traveled from its source.

  • radio waves: electromagnetic radiation with a wavelength between 1 × 10−3 and 1 × 105 meters; able to travel long distances without being broken up by atmospheric interference.

  • thermal radiation: electromagnetic radiation generated by charged particles in matter being moved around by heat; typically associated with infrared radiation and a frequency of 7 × 10−7 meters to 1× 10−3 meters.

  • ultraviolet (UV) radiation: electromagnetic radiation with more energy than visible light but less than x-rays, in the wavelength range of 4 × 10−7 to 1 × 10−7 meters.

  • visible light: electromagnetic radiation that human eyes can see, with a wavelength between 4 × 10−7 to 7 × 10−7 meters.

  • wavelength: the distance between crests or troughs of a wave.

  • x-ray: electromagnetic radiation with a wavelength between 1 × 10−10 and 1× 10−8 meters.

The Electromagnetic Spectrum

The energy transmitted through space from the sun to Earth, from a radio station to a radio, and from a television to an eye all fall somewhere on the electromagnetic (EM) spectrum. The EM lays out the various forms of radiation according to wavelength.

All electromagnetic energy is transmitted as a wave. Each wave has a crest (high point) and a trough (low point). Wavelength measures the distance between two crests or two troughs. This is the distance the wave moves in one complete cycle. The frequency of these cycles—waves per second—is measured in hertz. One hertz is equivalent to one cycle per second.

The energy carried by a wave is directly proportional to its frequency. Higher frequency means more energy; lower frequency, less energy. Furthermore, energy and frequency are both inversely related to wavelength. Longer wavelengths have less energy and lower frequencies. EM waves emitted from a single point (for example, a radio tower) lose their intensity according to an inverse-square law. The farther they travel from their point of origin, the weaker they get.

Maxwell, Rutherford, and Villard

The history of electromagnetic radiation in modern physics begins with Scottish physicist James Clerk Maxwell. In 1865, he published a theory to explain unifying electricity, magnetism, and light as a single phenomenon (known as EMR). He demonstrated that the interaction between electrical fields and magnetic fields can transmit energy through space as a wave. In the 1880s, German physicist Heinrich Hertz used Maxwell’s theory to prove the existence of radio waves.

In the late 1880s, physicists Ernest Rutherford and Paul Villard used ingenious experiments to detect and describe radioactive decay and the radiation it produces. Their work laid the groundwork for modern nuclear physics. Rutherford tested the penetrating power of the radiation from uranium. He detected the presence of alpha and beta particles. These particles transmit alpha and beta radiation. Meanwhile, Villard used similar experiments with radium and discovered an extremely penetrating form of EMR. Rutherford dubbed these gamma rays (also known as gamma radiation).

The electromagnetic spectrum is the range of all possible electromagnetic radiation wave-lengths. X-rays fall between gamma rays and UV light. Wavelengths for x-rays are between 0.1 and 10 nanometers, around the size of atoms. Radio waves can be as large as mountains, and gamma radiation has a short wavelength, smaller than atoms.

POP_Radiation_illustration.tif

Types of Radiation

Each type of electromagnetic radiation has unique properties that set it apart from others. These are described in ascending order of wavelength.

Gamma radiation has a wavelength of less than 1.0 × 10−11 meters. It comes from gamma decay. In this type of radioactive decay a photon (high-energy particle) is ejected from the nucleus of an atom. On Earth, this is emitted from radioactive isotopes of elements like radium. It may also occur as the result of the tremendous energy of a lightning strike.

X-rays have wavelengths between 1 × 10−10 and 1 × 10−8 meters. The wavelengths of x-rays are just small enough that they can pass through human flesh with little interference. They cannot pass through more dense materials such as bone. This is why x-rays are safe for medical use.

Ultraviolet radiation (UV rays or UV light) has a wavelength of 4 × 10−7 to 1 × 10−7 meters. The most familiar source of UV rays is the sun. UV rays are what cause skin tans or burns, as well as skin cancer.

Visible light (simply “light” in everyday speech) is the narrow band of EMR that human eyes can detect, with wavelengths between 4 × 10−7 to 7 × 10−7 meters. This range is further subdivided into all the colors of the rainbow, which correspond to specific wavelengths. For example, blue has the shortest wavelengths of visible light. Red has the longest. The “visible” designation is somewhat arbitrary, as several animals are able to see parts of the EMR spectrum humans cannot. For example, bees can see into the ultraviolet range.

Infrared has a wavelength of 7 × 10−7 meters to 1 × 10−3 meters and is closely associated with radiating heat (thermal radiation) that is invisible to the naked eye. This is why people can feel heat coming off their skin on a cold day. Some objects emit thermal radiation as visible light as well (objects so hot they glow).

Radio waves (which include microwaves) have wavelengths between 1 × 10−3 and 1 × 105 meters. These versatile waves are able to cover large areas of the earth at once and so have wide use as a method of communication. Radio waves can transmit information as variations in the waves. Radios, televisions, and other devices pick these up and turn them back into usable information. Any EMR with a longer wavelength than radio (greater than 1× 105 meters) is referred to by the generic term “longwave” radiation.

Radiation and Radioactivity

The term “radioactivity” was coined by Marie and Pierre Curie as a result of their intensive study of radioactive materials such as the ore called pitchblende. “Radioactivity” refers to a specific type of radiation, ionizing radiation, which occurs in the EMR at wavelengths shorter than those of visible light. An object or substance is radioactive only if it emits ionizing radiation. This is the dangerous radiation associated with nuclear bombs, uranium, and overexposure to the sun. In short, ionizing radiation has so much energy that it can remove electrons from atoms, including those of living things. Changing the atomic makeup of a living thing may have catastrophic results. These results can include an increased risk of cancer as a result of ionizing radiation damaging DNA and inducing mutations. Fortunately, ionizing radiation at low doses is essentially harmless. Life on Earth is subjected to some varying level of functionally harmless background radiation, including ionizing radiation, at all times.

Radiation’s Many Uses

EMR has become increasingly central to communications technology. EMR is used to send information to and from satellites as microwave radio waves, to broadcast television channels (including high definition) to satellite dishes and antennas in homes, to disinfect objects using UV radiation, and even to produce art by using paints that only reflect certain colors of light from the visible spectrum. Whether microwaving a meal, viewing a famous painting, or using x-rays to detect a broken bone, EMR makes possible the transmission of energy and information without physical contact.

Citation Types

MLA 9th
Vezina, Kenrick. "Radiation." Principles of Physics, edited by Donald R. Franceschetti, Salem Press, 2016. Salem Online, online.salempress.com/articleDetails.do?articleName=POP_0102.
APA 7th
Vezina, K. (2016). Radiation. In D. R. Franceschetti (Ed.), Principles of Physics. Salem Press. online.salempress.com.
CMOS 17th
Vezina, Kenrick. "Radiation." Edited by Donald R. Franceschetti. Principles of Physics. Hackensack: Salem Press, 2016. Accessed September 20, 2026. online.salempress.com.