# Background Essay: Tour of the Electromagnetic Spectrum: Microwaves

Electromagnetic Spectrum

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.

Microwaves

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.