## Tuesday, 13 January 2015

### Light as an Electromagnetic Wave

The history of radar begins with the history of our understanding that light is an electromagnetic waveIn 1826, André-Marie Ampère discovered that an electric current generated a magnetic field. Five years later, Michael Faraday discovered that an electric field is produced by a changing magnetic field. In 1855, Wilhelm Weber and Rudolf Kohlraush conducted an experiment to calculate the ratio of electromagnetic charge to electrostatic charge from direct measurements; the ratio was calculated to be 3.107×108 m/s. Only a few years before, Armand Fizeau and Léon Foucault had devised experiments to measure the speed of light, obtaining values of 3.149×108 m/s and 2.980×108 m/s, respectively. The significance of the discovery by Weber and Kohlrausch was not realized immediately, and for some time physicists believed it to be nothing more than a coincidence that this ratio agreed so closely with the speed of light. In 1861, James Clerk Maxwell published a correction to Ampère’s Law among his set of electrodynamic equations in On Physical Lines of Force. Ampere’s Law, in its original form, stated that a magnetic field was generated by an electric current. With Maxwell’s correction, it stated that a magnetic field was generated by a changing electric field – in essence, it was a corollary to Faraday’s Law. Starting from the equations published previously in On Physical Lines of Force, Maxwell published a mathematical derivation of the wave equation in his A Dynamical Theory of the Electromagnetic Field. This derivation proved that an accelerating electric field would generate a perpendicular magnetic field (and vice-versa), which together comprise an electromagnetic wave that can propagate through empty space. Solving Maxwell’s electromagnetic wave equation for the wave speed in vacuum reveals that such a wave would travel at the speed of light. Maxwell commented on the results of his derivations and the experiments of Weber and Kohlrausch, Fizeau, and Foucault, stating:
"The agreement of the results seems to show that light and magnetism are affections of the same substance, and that light is an electromagnetic disturbance propagated through the field according to electromagnetic laws" (Maxwell 1865).
Maxwell’s equations are at the foundation of our current understanding of optics, electrodynamics, and electric circuits. Maxwell’s electromagnetic wave theory also explains why light travels fastest in vacuum and must slow down when passing through a medium. The electromagnetic wave theory and Maxwell's equations form the theoretical foundation of all radar applications.

### Theory is put into Practice

Maxwell had predicted the existence of electromagnetic waves, but it was Heinrich Hertz who demonstrated that radio waves existed and could be transmitted, refracted, and reflected in the same manner as visible light. Alexander Popov, in 1897, while testing his apparatus to detect lightning strikes, observed interference when a ship had passed. Though Popov reported that the phenomena could possibly be exploited to detect objects, he did not explore this further. In 1904, Christian Hülsmeyer used radio waves to detect the presence of ships, but not their range or bearing. In September of 1922, U.S. Navy researchers Albert Taylor and Leo Young, like Popov before them, observed that a passing ship interrupted their radio communication. Taylor and Young realized the potential application and suggested radio transmitters and receivers be used to detect ships in low visibility. However, it wasn’t until Lawrence Hyland observed in 1930 that an airplane flying overhead interrupted radio communication did the U.S. military take serious interest in detecting objects using radio waves. The acronym RADAR, which stands for RAdio Detection and Ranging, was coined in 1934.

Walter Stern, possibly aware of the work of Hülsmeyer, developed the first ground penetrating radar (GPR) and used it to survey a glacier in Austria in 1929. The use of radio waves for subsurface mapping was essentially forgotten for several years, until a few airplanes belonging to the U.S. Air Force gave false altitude readings and the pilots crashed while trying to land on ice in Greenland. The renewed interest sparked investigations into the use of radar to map ice, groundwater tables, and subsoil properties. A GPR system essentially the same as the one used by Stern in 1929 was developed to investigate the lunar subsurface for the Apollo 17 mission. GPR first became commercially available in 1972, and since then there has been much research into the technology and its applications. Today, GPR is used in a wide variety of non-destructive, subsurface mapping applications, including:

• detecting buried explosives
• locating possible archaeological dig sites
• locating buried pipes
• locating embedded reinforcing steel
• inspecting pavements
• mapping soil strata
• mapping contaminant plumes
• mapping groundwater levels
• mapping ice thicknesses

### References

Annan, A. P. (2009). Electromagnetic principles of ground penetrating radar. In Ground Penetrating Radar: Theory and Applications. Edited by Jol, H. M. Elsevier, Amsterdam, Netherlands.

Cassidy, N. J. (2009). Electrical and magnetic properties of rocks, soils and fluids. In Ground Penetrating Radar: Theory and Applications. Edited by Jol, H. M. Elsevier, Amsterdam, Netherlands.

Clarke, G. K. C. (1987). A short history of scientific investigations on glaciers. Journal of Glaciology, special issue: 4-24.

Crease, R. P. (2008). The great equations: breakthroughs in science from Pythagoras to Heisenberg. W. W. Norton & Company, Inc., New York, New York.

Guarnieri, M. (2010). The early history of radar. IEEE Industrial Electronics Magazine, 4(3): 36-42.

Jol, H. M. (2009). Preface. In Ground Penetrating Radar: Theory and Applications. Edited by Jol, H. M. Elsevier, Amsterdam, Netherlands.

Keithley, J. F. (1999). The story of electrical and magnetic measurements: from 500 B.C. to the 1940s. Institute of Electrical and Electronics Engineers, Inc., New York, New York.

Kostenko, A. A., Nosich, A. I., and Tishchenko, I. A. (2001). Radar prehistory, Soviet side: three coordinate L-band pulse radar developed in Ukraine in the late 30's. Proceedings of the IEEE Antennas and Propagation Society International Symposium, Boston, Massachusetts, 8-13 July 2001. Institute of Electrical and Electronics Engineers, New York, New York. 4: pp. 44-47.

Maxwell, J. C. (1861). On physical lines of force [online]. Philosophical Magazine and Journal of Science. Available from http://goo.gl/nfk1Fk [last accessed 13 January 2015].

Maxwell, J. C. (1865). A dynamical theory of the electromagnetic field [online]. Philosophical Transactions of the Royal Society, 155: 459-512. doi: 10.1098/rstl.1865.0008.

Olhoeft, G. R. (1996). Application of ground penetrating radar. Proceedings of the 6th International Conference on Ground Penetrating Radar, Sendai, Japan, 30 September - 3 October 1996. Institute of Electrical and Electronics Engineers, New York, New York. pp. 1-4.

Olhoeft, G. R. (2002). Applications and frustrations in using ground penetrating radar. IEEE AESS Systems Magazine, 17(2): 12-20.

Page, R. M. (1962). The early history of radar. Proceedings of the Institute of Radio Engineers, 50(5): 1232-1236.

Stern, W. (1929). Versuch einer elektrodynamischen dickenmessung von gletshereis. Gerlands Beitrge zur Geophysik, 23: 292-333.

Young, H. D. and Freedman, R. A. (2004). Sears and Zemansky's university physics: with modern physics, 11th edition. Pearson Addison Wesley, San Francisco, California.

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