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.
With a name like “microwave oven,” the appliance in your kitchen no doubt uses microwaves! Beyond being able to cook food, though, microwaves have wide applications for communications, remote sensing, GPS tracking, astronomy, and other technologies. Microwaves have more energy (and therefore shorter wavelengths) than radio waves, but not quite as much as visible light does. Their wavelengths range from 1 mm to 30 cm. The microwaves that your microwave oven uses to cook your food have a wavelength of about 12 cm. They excite water and fat molecules, and the resultant motion heat up your food.
Within the microwave region of the electromagnetic spectrum, there are several sub-types, categorized based on how the various energy ranges are used. Some ranges of microwave energy are able to easily pass through the atmosphere, dust, snow, and rain to produce clear images of the Earth’s surface. Microwave imaging from space helps scientists track and observe everything from shifting sea ice to deforestation to weather phenomena such as hurricanes. Forecasting and weather prediction depend on microwaves, too. Such systems use active radar, or Doppler Radar, which involves a microwave sensor sending out a signal and then comparing the broadcasted and reflected signals to produce a dynamic image. Microwave sensing is also used to track changes in sea height, an important measurement related to how much heat is stored in the world’s oceans.
The discovery of universal, omni-directional background microwave energy “noise” helped scientists understand the Big Bang. By studying the energy echo left over from the dramatic emergence of the universe, scientists were able to learn more about its origins.
NARRATION: Microwaves can pop your popcorn. They can catch you speeding. They carry thousands of phone channels to speed your calls. But, can microwaves help us learn about our world and our universe? Let’s find out.
With wavelengths ranging from 30 centimeters down to 1 millimeter, microwaves fall between radio waves and infrared. Microwaves are used in Doppler radar, which is widely used for short term localized weather forecasting and what you see on TV weather news. Satellites have revolutionized weather forecasting by providing a global view of weather patterns and surface temperatures. This unique perspective has greatly increased the accuracy of tropical storm and climate forecasts.
Different wavelengths of microwaves, grouped into bands, provide different information to scientists. Medium length C band microwaves penetrate through clouds, dust, smoke, snow, and rain to reveal the Earth’s surface. Satellite microwave measurements reveal the full Arctic sea ice cover every day, even where clouds exist. These measurements show great variability from year to year, but, also an overall decrease in Arctic sea ice since the late 1970s, illustrated here with maps and a time series of Arctic sea ice in September, at the end of the summer melt.
The Japanese Earth Resources satellite uses longer wavelength L band microwaves for forest mapping by measuring surface soil moisture, such as this image of the Amazon basin, to identify areas of recent deforestation. L band microwaves are also used by global positioning systems, such as the one in your car.
Scientists routinely combine microwave information with information from other parts of the EM spectrum to study the composition of cosmic dust or of a supernova, such as this supernova image that combines X-ray, radio, and microwave data. This recently known supernova in the Milky Way exploded just over 140 years ago at the time of the American Civil War.
One important phenomenon is unique to microwaves. In 1965, using long L band microwaves, Arno Penzias and Robert Wilson made an incredible accidental discovery. They detected what they thought was noise from their instrument, but was actually a constant background signal coming from everywhere in space. This radiation is called cosmic microwave background, and if our eyes could see microwaves, the entire sky would glow with a nearly uniform brightness in every direction. The existence of this background radiation has served as important evidence supporting the Big Bang Theory for how our universe began.
Microwaves have become both staples and wonders of modern life. They are also the backbone of communications and of Earth sensing systems, and they are an excellent guide to the ancient history and origins of our universe.
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