What is the Electromagnetic Spectrum?
The visible light or radiant energy that illuminates our portion of the universe and enriches our existence with the appearance of different colors and hue intensities, was the first type of electromagnetic radiation evident to mankind. The remaining regions of the electromagnetic spectrum have only recently been elucidated. These varied regions can be differentiated on the basis of their wavelength (in length units of meters or millimeters), or frequency (in sec−1 or Hertz units).
The first region other than the section of the spectrum visible to human eyes was the infrared portion. William Herschel, also known as the discoverer of the first planet to be revealed in modern times, Uranus, was responsible for slowing rays of light with a prism, and redirecting the light rays into heat-absorbing bulbs. He found that the “caloric rays” were most intense beyond the red portion, producing the highest absorption temperatures yet the rays could be refracted and reflected like visible light.
In Germany, Johann Ritter, learning about Herschel’s discovery, attempted to identify the complementary radiation beyond the violet region of the visible spectrum by exposing silver chloride crystals to refracted sunlight. Ritter originally called this new discovery “chemical radiation” but in time, this radiation became known as ultraviolet (beyond the violet).
James Clerk Maxwell created the Electromagnetic Theory, which served to unify the initially disparate fields of electricity and magnetism utilizing Maxwell’s Equations. His work suggested that light itself was one of several types of electromagnetic waves, all traveling at the velocity of light, c.
The next portion of the electromagnetic spectrum to be identified was located in the low energy region. In 1887, German physicist Heinrich Hertz added very long wavelength radio waves to the spectrum, but his research did not pursue applications of this technology as he felt that there was no practical use for it. It was left to Nicola Tesla and Guglielmo Marconi to find ways to utilize “wireless telegraphy” for the public.
The discovery of X-rays followed soon thereafter. Wilhelm Roentgen, a Bavarian physicist, studied the passage of cathode rays from an induction coil through a glass tube that had been partially evacuated. He noticed that these rays when projected upon a fluorescent screen caused it to glow. Roentgen also found that these rays penetrated skin and could cast an image of the bones within upon on photographic plate. The first X-ray image published was that of Frau Roentgen’s hand.
Interest in uncovering new elements and new phenomena such as X-rays was all consuming as the end of the nineteenth century approached. Henri Becquerel, in Paris, discovered that uranium salts were the source of radioactivity. Another Parisian researcher, Paul Villard, also studied radioactive sources and in 1900, established that certain radioactive materials emitted what become known as gamma rays, high-energy radiation with even shorter wavelengths than X-rays.
By the early twentieth century, most of the regions of the electromagnetic spectrum had been explored and applications of the different manifestations such as radio waves and X-rays had been explored. One region, however remained largely unexamined until the 1940s. This type of electromagnetic radiation, initially known as ultrashort radio waves, consisting of wavelengths in the 1 meter – one millimeter range, was used to send radar signals to establish distance. Use of this application expanded during World War II. Engineers at the Raytheon corporation building the vacuum tubes for military uses noticed that the heat emitted by the tubes could be used to warm their hands in the winter months. The idea of incorporating this technology to construct microwave ovens was implemented by Raytheon engineers John Spencer and Marvin Bock. The ubiquitous modern cell phones also utilize microwave radiation to send signals, at an intensity level too low to result in thermal heating.
New applications are continually being added to the complement of uses for the different ranges of wavelengths and frequencies encompassed by the electromagnetic radiation, shedding “light” on previously unexplored areas of potential technology.