The application of remote sensing sensors and methods relies on the existence of electromagnetic (EM) radiation. Whether it is being measured through the emissions of an object (reflection of sunlight) or whether radiation is actively sent from an instrument, without this source of energy, we cannot analyze a signal. (LM-Electromagnetic Waves and their Spectrum)
But what are the waves?
Mechanical waves and electromagnetic waves are two important ways that energy is transported in the world around us.
Waves in water and sound waves in the air are two examples of mechanical waves. Mechanical waves are caused by a disturbance or vibration in matter, whether solid, gas, liquid, or plasma. The matter that waves are traveling through is called a medium. Water waves are formed by vibrations in a liquid and sound waves are formed by vibrations in a gas (air). (EiS)
Electricity can be static, like the energy that can make your hair stand on end. Magnetism can also be static, as it is in a refrigerator magnet.
A changing magnetic field will induce a changing electric field and vice-versa—the two are linked. These changing fields form electromagnetic waves.
As visualized on the right, an EM wave consists of an electrical field (EF, red) and a magnetic field (MF, blue). The EF varies in magnitude in the direction that is perpendicular to that of the travel direction. A corresponding MF is oriented at right angles to the EF. Both fields travel at the speed of light (~300,000 km/s).
Electromagnetic waves differ from mechanical waves in that they do not require a medium to propagate. This means that electromagnetic waves can travel not only through air and solid materials, but also through the vacuum of space.
A history detour: EM waves
In the 1860’s and 1870’s, the Scottish scientist James Clerk Maxwell developed a scientific theory to explain EM waves. He noticed that electrical fields and magnetic fields can couple together to form electromagnetic waves. He summarized this relationship between electricity and magnetism into what is now referred to as “Maxwell’s Equations“.
Heinrich Hertz, a German physicist, applied Maxwell’s theories to the production and reception of radio waves. The unit of frequency of a radio wave — one cycle per second — is named the hertz, in honor of Heinrich Hertz.
His experiment with radio waves solved two problems. Firstly, he demonstrated concretely, what Maxwell had only theorized — that the velocity of radio waves was equal to the velocity of light. This proved that radio waves are a form of light. Secondly, Hertz figured how to make the electric and magnetic fields detach themselves from wires and go free as Maxwell’s waves — electromagnetic waves. (LM)
watch the video from Prof. Ian Woodhouse
How do we describe an EM wave?
The terms light, electromagnetic waves, and radiation all refer to the same physical phenomenon: electromagnetic energy. This energy can be described by frequency (f), wavelength (λ) and the speed of light (c). All three are related mathematically such that if you know two of these, you can calculate the missing third variable using this equation:
c = \lambda f
Thus, EM waves can be described through the characteristics of two parameters: a) wavelength and b) frequency. Both are closely correlated (inversely) and depend on each other.
Electromagnetic waves have crests and troughs similar to those of ocean waves. The distance between crests is referred to as ‘wavelength’ and is denoted with the symbol λ. The shortest wavelengths are just fractions of the size of an atom, while the longest wavelengths scientists are currently studying can be larger than the diameter of our planet.
The number of crests that pass a given point within one second is described as the frequency of the wave. One wave or cycle per second is called a Hertz (Hz), after Heinrich Hertz who established the existence of radio waves. A wave with two cycles that pass a point in one second has a frequency of 2 Hz.
The amplitude is another important variable that characterizes the distance between the peak and trough of a wave. With these three measures (wavelength, amplitude, frequency), you are now able to describe an EM wave.
Explore the unique features of an EM wave
This interactive tool gives you a hands-on experience of the frequency and the amplitude of a wave, two very important parameters. Try the sliders below to get a feel of what these parameters actually mean.
The spectrum of EM radiation
Electromagnetic energy travels in waves and spans a broad spectrum from very long radio waves to very short gamma rays. The human eye can only detect only a small portion of this spectrum called visible light. A radio detects a different portion of the spectrum, and an x-ray machine uses yet another portion. Scientific instruments use the full range of the electromagnetic spectrum to study the Earth, the solar system, and the universe beyond.
The graphic above demonstrates the different portions of the electromagnetic spectrum, including their names, wavelengths, and frequencies. It also depicts the scale of objects/organisms which can interact with the given wavelength and how life is influenced by them. Keep in mind, that every portion of light, whether it is short- or long waved, corresponds to a distance, that we could measure to indicate why certain objects are more influenced by it than others.
Sources & further reading
AWF-Wiki (2013). Electromagnetic Radiation. <http://wiki.awf.forst.uni-goettingen.de/wiki/index.php/Electromagnetic_radiation>
Centre for Remote Imaging, Sensing & Processing (CRISP) (2001). Principles of Remote Sensing. <https://crisp.nus.edu.sg/~research/tutorial/rsmain.htm>.
Elachi, C. & van Zyl, J. (2015²). Introduction to the Physics and Techniques of Remote Sensing. Hoboken, USA: John Wiley & Sons, Inc.
European Space Agency (ESA). (2019). Educational Support. The Electromagnetic Spectrum. <https://sci.esa.int/web/education/-/50368-the-electromagnetic-spectrum>.
Jensen, J.R. (2007²). Remote Sensing of the Environment. An Earth Resource Perspective. Upper Saddle River, USA: Pearson Prentice Hall.
Lumen Learning. Waves and Wavelengths. <https://courses.lumenlearning.com/atd-bhcc-intropsych/chapter/waves-and-wavelengths/>
Rees, W.G. (2010²). Physical Principles of Remote Sensing. Cambridge, USA: Cambridge University Press.
Schowengerdt, R.A. (2007³). Remote Sensing. Models and Methods for Image Processing. San Diego, USA: Academic Press.