Electromagnetic spectrum

The electromagnetic spectrum is a measure of the full range of electromagnetic radiation—a type of energy more commonly known as light. The electromagnetic spectrum gauges electromagnetic radiation by wavelength and frequency. At one end of the spectrum are longer wavelength, lower frequency radio waves; at the other are shorter wavelength, higher frequency gamma rays. Most of this radiation is invisible to the human eye and can only be detected with special instrumentation. The visible light capable of being perceived by humans is just a small part of the electromagnetic spectrum.

Background

Electromagnetic radiation can be produced by processes involving changing electric and magnetic fields, including the acceleration of charged particles and transitions of electrons in atoms. The moving electrons create an oscillating electric and magnetic field that travels as a particle of light energy called a photon. Photons move at incredibly high speeds of exactly 186,282 miles per second (299,792,458 kilometers per second), a measure commonly referred to as the speed of light.

The oscillating electromagnetic radiation moves in the form of waves, with peaks and valleys, similar to waves of water on the ocean. The distance between each wave peak is called the wavelength, and it is measured in meters. The number of wave cycles—a full wave from peak to peak—that pass a given point over a length of time is called the frequency. Frequency is most often measured by the number of wave cycles per second, a unit known as hertz (Hz). Waves with longer wavelengths have lower frequencies because fewer wave peaks cycle per second. Conversely, waves with shorter wavelengths have higher frequencies because more peaks can cycle per second. Radiation with shorter wavelengths and higher frequencies contains more energy than longer wavelengths and lower frequencies.

Overview

Scholars as far back as the thirteenth century theorized that the familiar colors of a rainbow were caused by sunlight reflecting through droplets of water. In the 1660s, famed British physicist Sir Isaac Newton passed a beam of sunlight through a glass prism and showed that white light could be separated into seven spectral colors: red, orange, yellow, green, blue, indigo, and violet. His experiments proved that white light was made up of a mixture of colors that could be separated by the prism. Newton referred to this phenomenon as the spectrum, a Latin term meaning “appearance” or “apparition.”

In the year 1800, British astronomer Sir William Herschel was attempting to determine the amount of heat generated by each color in the spectrum. During his experiments, he noticed that a thermometer left just outside the edge of the red band registered the highest temperature. The invisible heat radiation Herschel discovered was later named infrared, or “below the red.” A year later, German physicist Johann Wilhelm Ritter discovered that an invisible form of light existed beyond the violet edge of the spectrum. This light was later named ultraviolet, or “beyond violet.”

In 1887, German physicist Heinrich Hertz proved the long-theorized existence of light with longer wavelengths than infrared when he produced radio waves. The frequency measurement, the hertz, was named in his honor. In 1895, German scientist Wilhelm Conrad Röntgen discovered a form of high-frequency radiation that he labeled X-rays to indicate an unknown type of radiation. Five years later, French physicist Paul Villard discovered a form of even higher energy radiation that was later called gamma rays.

During the early study of high-energy radiation, scientists were unsure of the nature of this high-energy radiation; they only later determined it was another form of light. This range of light radiation is called the electromagnetic spectrum and is divided into seven regions.

Radio waves have the longest wavelengths and lowest frequencies in the spectrum. Their wavelengths can stretch from 100 kilometers to 1 millimeter; radio wave frequencies can range from 3,000 wave cycles per second, or 3 kilohertz (kHz), to 300 billion wave cycles per second, or 300 gigahertz (GHz). Radios, cell phones, televisions, and global positioning systems (GPS) all use different frequencies of radio waves. For example, cell phones use several licensed radio-frequency bands, including low- and mid-band spectrum and, for some 5G service, millimeter-wave bands such as 24, 28, 37, 39, and 47 GHz.—a megahertz corresponds to one million wave cycles per second. Cable television operates between 54 and 1,000 MHz. Atmospheric lightning emits radio waves at a frequency of .003 to 30 kHz, which is why it can sometimes be heard on AM radio transmissions.

Microwaves are considered a subclass of radio waves with higher energies. Microwave frequencies range between 300 MHz and 300 GHz. Microwaves are commonly used in homes as a fast cooking method, but they are also used in communication systems and radar. A typical microwave oven operates at a frequency of 2.45 GHz, while a police radar gun uses frequencies between 33.4 and 36 GHz.

Infrared is also referred to as heat radiation. Infrared wavelengths range from about 1 millimeter to 750 nanometers—750 billionths of a meter—and have a frequency range of 300 GHz to 400 trillion hertz, or 400 terahertz (THz). Living organisms emit infrared radiation, which can be detected by devices such as night-vision goggles. Security systems and TV remote controls also use infrared radiation. Infrared astronomy became especially prominent after the James Webb Space Telescope released its first science images in 2022, showing how infrared light can reveal distant galaxies and objects hidden by dust.

Visible light is the narrow band of the electromagnetic spectrum that can be detected by the human eye. It ranges from red light, with wavelengths of 620 to 750 nanometers and frequencies of 400 to 484 THz, to violet, with wavelengths of 380 to 450 nanometers and frequencies of 688 to 750 THz.

Ultraviolet is a higher-energy radiation than visible light. Its wavelengths fall between 10 and 400 nanometers, and its frequency between 750 THz and 30 petahertz (PHz)—a figure representing 30 quadrillion hertz. It is so energetic that it can begin to break down some chemical bonds. Prolonged exposure to ultraviolet radiation can damage human skin, causing sunburn.

X-rays are produced by the acceleration of high-energy electrons. They have wavelengths of .01 to 10 nanometers and frequencies higher than 30 PHz. Lower energy X-rays are often used in the medical field for internal imaging, such as computed tomography (CT) scans. Higher energy X-rays can penetrate matter and are similar to gamma rays. Because CT scans use X-rays, US quality programs track excessive radiation dose while preserving diagnostic image quality.

Gamma rays are an extremely high-energy form of radiation with wavelengths the size of atoms and frequencies measured in scientific notation as greater than 10²⁰ hertz. Gamma rays are produced in the nuclei of atoms and are damaging to living tissue. In small, controlled doses, gamma rays are used to treat cancer.

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