The electromagnetic (EM) spectrum is the range of types, or frequencies, of EM radiation. The EM spectrum ranges from shorter wavelengths, including gamma and x-rays, to longer wavelengths, including microwaves and broadcast radio waves. The part of the EM spectrum that can be observed is known as the visible spectrum and includes the common wavelengths of what is perceived as color.
Electromagnetic radiation can be expressed in terms of energy, wavelength, or frequency. Frequency is measured in cycles per second, or hertz. Wavelength is measured in meters. And energy is measured in electron volts. These three quantities for describing EM radiation are related to each other, and the radiation is generally measured and expressed in the smallest number possible.
The sun is a source of all of the types of radiation across the electromagnetic spectrum and its EM radiation bombards the atmosphere, which protects the Earth from exposure to the range of higher energy waves that can be harmful to life. This includes the radiation such as gamma or x-rays and some ultraviolet waves, which have such high energy that they can knock electrons out of atoms, alter atoms and molecules, and cause damage to cells in organic matter. In healthcare, these same rays are used for imaging or for killing cancer cells.
The electromagnetic spectrum, when expressed from longest wavelength to shortest, includes common types of EM radiation—such as radio waves, which are used by radio stations and are emitted by stars and gases in space. Most of the radio wave part of the EM spectrum falls in the range of about 1 cm to 1 km in wavelength and is also expressed as 30 gigahertz to 300 kilohertz in frequency. This is the broadest part of the spectrum. Microwave radiation is commonly used for cooking food and has also been used by astronomers to learn about the structure of nearby galaxies. Infrared can be used to detect light emitted by skin and objects with heat and can help map the dust between the stars in space. Visible radiation is the radiation detected as light and includes the light emitted by fireflies, lightbulbs, and stars. Both infrared and visible radiation are often expressed in wavelength. For the visible radiation expressed as color, the wavelengths can be expressed using nanometers; the colors violet, blue, green, yellow, orange, and red have wavelengths between 400 to 700 nanometers.
Ultraviolet radiation is commonly referred to as UV and is the reason skin tans and burns during sunny days. Hot objects in space also emit UV radiation. X-ray radiation is commonly used to image unseen things, such as bone through skin and for airport security, and is also emitted by hot gases in space. Gamma rays are also used for imaging and are generated by the universe at large. The wavelengths of ultraviolet, x-ray, and gamma-ray regions of the EM spectrum are small. Instead of wavelengths, this portion of the EM spectrum is often referred to by their energies, which are measured in electron volts. Ultraviolet radiation falls in the range from a few electron volts to about 100 eV. X-ray photons have energies in the range of 100 eV to 100,000 eV. And gamma rays are all photons with energies greater than 100 keV.
Types of electromagnetic radiation
Extremely high frequency
Super high frequency
Ultra high frequency
1 nm to 100 pm
300 PHz to 3 EHz
1.24 keV to 12.4 keV
1 μm to 100 nm
Near ultraviolet, visible light
300 THz to 3 PHz
1.24 eV to 12.4 eV
Very high frequency
Very low frequency
Ultra low frequency
Super low frequency
Extremely low frequency
Although all electromagnetic waves travel at the speed of light in a vacuum, they do so at a range of frequencies, wavelengths, and photon energies. This spectrum comprises the range of all electromagnetic radiation and consists of several subranges, commonly referred to as portions, such as visible light or ultraviolet radiation. The various portions bear different names based on differences in behavior in the emission, transmission, and absorption of corresponding waves, and based on their practical applications. Because there are also no precise accepted boundaries between any of the portions, the ranges tend to overlap.
Electromagnetic radiation is reflected or absorbed by many gases in the Earth's atmosphere—the more important gases being water vapor, carbon dioxide, and ozone. Some radiation, such as visible light, largely passes through the atmosphere. The regions of the spectrum with wavelengths capable of passing through the atmosphere are referred to as "atmospheric windows" with some microwaves capable of passing through clouds, which has made this part of the spectrum the best wavelength for certain communication systems, especially terrestrial to satellite communications.
Astronomers use the entire EM spectrum for a variety of use cases based on the wavelength and its related capabilities. For example, radio waves and microwaves, the longest wavelength and lowest energies of the EM spectrum, have been used to view dense interstellar clouds and track the motion of cold, dark gas. Telescopes using radio waves have been used to map the structure of the galaxy, and telescopes using microwaves have been used to detect the remnant glow of the big bang.
Meanwhile, telescopes using infrared waves have been used to find cool, dim stars, and measure the temperatures of planets in other solar systems. The wavelengths of infrared radiation are long enough to navigate through clouds that would otherwise block other types of radiation, such as visible light. As well, as the color of a star can help astronomers identify how hot it is, and with the majority of stars emitting most of their electromagnetic energy as visible light, infrared telescopes have been used to identify those temperatures and find the coldest of stars, which barely emit any visible light.
Astronomers also use ultraviolet radiation, or UV, to find energetic stars and identify regions of star birth through space. This is possible as with UV telescopes, most of the stars and gas are hidden from view and the energy activity around new or energetic stars is easier to see. Beyond this, x-rays and gamma rays, which are blocked by the atmosphere, are used in space-based telescopes to identify other phenomenon in space.
X-rays are emitted and can be used to detect neutron stars, vortexes of superheated material spiraling around a black hole, or diffuse clouds of gas in galactic clusters. Meanwhile, the shortest of the wavelengths, gamma rays, are deadly to humans and have been used to detect supernova explosions, cosmic radioactive decay, and the destruction of antimatter. Gamma ray bursts, a brief flickering of gamma ray light from the explosion of a star and creation of a black hole, are among the most energetic events in the universe.
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