The electromagnetic – or EM – spectrum is made up of seven kinds of electromagnetic energy with each corresponding to a different range. From lowest energy to highest energy, the seven groupings along the spectrum are: Radio Waves, Microwaves, Infrared, Visible Light, Ultraviolet, X-rays, and Gamma Rays. Electromagnetic energy travels in waves and spans a broad spectrum from very long radio waves to very short gamma rays. Read in the order listed above, waves increase in frequency and decrease in wavelength. The visible range, which is the only portion of the spectrum the human eye can detect, represents only a very tiny portion of the entire electromagnetic spectrum.
All electromagnetic radiation is made up of up of fields of electricity and magnetism interacting with each other. Electric fields can be static – like the static electricity that can hold a balloon to the wall. Magnetic fields can be static, too – like what holds a refrigerator magnet in place. However, electric and magnetic fields can also change and move together, and when that happens, the interaction produces waves: electromagnetic waves. EM energy can be described by frequency, wavelength, or energy, all of which are inter-related by the expression E = Frequency / Wavelength. Frequency is directly proportional to energy (they increase and decrease together) while wavelength is inversely proportional to energy (as wavelength increases, energy decreases).
Radio and microwaves are usually described by frequency (units of Hertz), infrared and visible light by wavelength (units of meters), and x-rays and gamma rays by energy (units of electron-volts). Though referred to by different names – light, EM radiation, or rays – all EM energy is made up of the same kinds of waves. The convention of using different units for different parts of the spectrum is simply a convenience that has to do with using numbers that are neither too large nor too small. The distinctions between the energy bands are simply a convention that eases communication. The EM spectrum doesn’t actually have breaks or chapters.
When you think of a water wave in the ocean, it might be easy to imagine the water oscillating up and down, creating a traveling waveform across the water. Even easier to imagine: making waves travel along a jump-rope secured to a wall at one end. In that case, it’s easy to see that the wave’s oscillation is perpendicular to the direction of its forward movement. In other words, the movement of the rope may be up and down, but the wave that travels through the rope is moving forward or backward—two perpendicular directions. These kinds of waves are called “transverse” waves. In transverse waves, the direction of the wave is perpendicular to the direction of applied energy. Another type of wave is a “longitudinal” wave, in which the wave moves parallel to the applied energy. With sound waves and other longitudinal waves, molecules vibrate and bump into one another, passing energy along the same direction the wave is moving.
While some transverse waves and some longitudinal waves might be easy to imagine, electromagnetic wave are harder to visualize. Because the wave is traveling in a direction that’s perpendicular to both the electric field and the magnetic field, thinking about EM waves requires three-dimensional visualization. Transverse waves like the jump-rope example give a close approximation, but electromagnetic radiation is more complex. One important feature of EM radiation is that, since its movement is based on the interaction of electric and magnetic fields, and electric and magnetic forces are possible over long distances, EM waves can travel through a vacuum. No material medium is necessary. Remember that low-energy EM radiation has longer wavelengths, corresponding to lower frequencies. High-energy EM radiation has shorter wavelengths, corresponding to higher frequencies.
All EM radiation travels at the same speed: the speed of light.
The categories along the spectrum – Radio, Microwave, Infrared, Visible, Ultraviolet, X-ray, Gamma Ray - represent a useful breakdown of EM radiation that helps scientists understand and visualize energy sources on, in, and under the Earth as well as throughout our solar system, galaxy, and universe.
EM radiation in the radio band of the spectrum includes the longest wavelengths, from about the size of a water bottle to the size of a car, ship, mountain, even our Earth. This long-wavelength radiation has the lowest energy along the EM spectrum. Its existence was proven in 1888 by Heinrich Hertz, whose name you might recognize as the unit for frequency, or cycles per second. Hertz showed that the energy he had detected was the same as other known electromagnetic waves – showing that radio waves were a form of EM radiation. In 1932, Karl Jansky showed that stars and other objects in space emit radio waves, and, later, scientists proved that any astronomical object with a changing magnetic field can produce radio waves.
Radio telescopes are much larger than optical telescopes – because radio waves have much longer wavelengths. In order to get a clear, high-resolution radio image, astronomers often use an array of radio telescopes (sometimes arranged in a “Y” shape) that, together, act like one large telescope. At radio frequencies, the sky appears very different than it does in the visible band of the EM spectrum. Pulsars, supernova, emerging stars, and quasars dominate the radio sky, while, at energies corresponding to visible light, point-like stars tend to dominate.
Here on Earth, radio waves are used primarily for communications. Categorized by long-wave, medium-wave, VHF (very high frequency) and UHF (ultra-high frequency), radio communications include local radio stations, broad-coverage stations, and – at the shorter-wavelength end of the radio band – police and military communications. Other uses include GPS receivers, garage door openers and radio-controlled toys.
NARRATION: Guglielmo Marconi’s first radio transmissions in 1894 have spread into space for over 100 years at the speed of light. They passed Sirius in 1903, Vega in 1919, and Regulus in 1971. That signal has already passed over 1 thousand stars. Anyone orbiting one of those stars, with a really good receiver, could detect Marconi’s signal and know that we are here.
Radio waves are the longest and contain the least energy of any electromagnetic wave. While visible light is measured in minute fractions of an inch, radio waves vary from about 19 centimeters, about the length of a water bottle, to waves the length of cars, ships, mountains, all the way up to monstrous waves longer than the diameter of our planet.
Heinrich Hertz discovered radio waves in 1888. The first commercial radio station went on the air in Pittsburgh, Pennsylvania on November 2, 1920. Then, in 1932, a major discovery by Karl Jansky at Bell Labs revealed that stars and other objects in space radiated radio waves. Radio astronomy was born.
However, scientists need giant antennas to detect weak long wavelength radio waves from space. The enormous Arecibo radio dish antenna measures 305 meters in diameter, over 3 football fields. Scientists can link the signals from an array of separate radio antennas to focus on tiny slices of distant space. Such arrays act as a single immense collector. This giant New Mexico array uses 27 parabolic dish antennas shaped into a giant Y, with each arm capable of stretching 13 miles. Scientists have even spread these linked antennas across the globe. One of the largest stretches from Hawaii to the Virgin Islands, and acts like such a powerful telephoto lens that a baseball sitting on the Moon would fill its entire field of view.
Many of the greatest astronomical discoveries have been made using radio waves. Pulsars, the existence of giant clouds of superheated plasma, which are among the largest objects in the universe, and even quasars, such as this one over 10 billion light years away, were all discovered using radio waves.
Radio waves also provide more local information. Astronomical objects that have a magnetic field usually produce radio waves, such as our Sun. Thus, NASA’s STEREO satellite is able to monitor bursts of radio waves from the Sun’s corona. Wave sensors on the WIND spacecraft record the radio waves emitted by a planet’s ionosphere, such as the bursts from Jupiter, whose wavelength measures about 15 meters.
Radio waves fill the space around us to bring entertainment, communications, and key scientific information. We can’t hear these radio waves. When you tune your radio to your favorite station, the radio receives these electromagnetic radio waves and then vibrates a speaker to create the sound we hear. We may not be able to tap our toes to the cosmic radio transmissions, but we certainly discovered much about our universe’s grand cosmic dance by listening to them.
Academic standards correlations on Teachers' Domain use the Achievement Standards Network (ASN) database of state and national standards, provided to NSDL projects courtesy of JES & Co.
We assign reference terms to each statement within a standards document and to each media resource, and correlations are based upon matches of these terms for a given grade band. If a particular standards document of interest to you is not displayed yet, it most likely has not yet been processed by ASN or by Teachers' Domain. We will be adding social studies and arts correlations over the coming year, and also will be increasing the specificity of alignment.