The theory of electromagnetism was developed by physicist James Clerk Maxwell. According to his theory, an alternating current would set up fluctuating electric and magnetic fields in the region surrounding the original disturbance. Furthermore, these waves were predicted to have a frequency equal to the frequency of the current oscillations. Moreover, Maxwell theory predicted that the radiated waves would behave in every way like light i.e. electromagnetic waves would be reflected by metal mirrors, would be refracted by dielectrics like glass, would exhibit polarization and interference and would travel outward from the wire through a vacuum with a speed of 3.0 x 108 m/s. This led to the unifying and simplifying assumption that light was also a type of Maxwell wave or electromagnetic disturbance, created by extremely high frequency electric oscillators in matter.
Heinrich Hertz in a series of experiments showed that Maxwell’s theory was correct and that an oscillating electric current does indeed radiate electromagnetic waves that possess every characteristic of light except the same wavelength as light. Using a simple spark gap oscillator consisting of two short stubs terminated in small metal spheres separated by an air gap of about half an inch, he applied pulses of high voltage, which caused a spark to jump the gap and produce a high frequency electric oscillation of about 5 x 108 Hz. These oscillations, or ringing, occur while the air gap remains conducting and charges surge back and forth between the spheres until electrical equilibrium is established. He used a simple loop antenna with a small spark gap as the receiver. Heinrich Hertz succeeded in detecting the radiation from his spark gap oscillator, even at distances of several hundred meters. Furthermore, he found the detected radiation to have a wavelength of about 60 cm, matching the oscillator frequency of 5 x 108 Hz, note c = λf , where λ is the wavelength and f is the frequency.
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Heinrich Hertz proceeded to show that these electromagnetic waves could be reflected, refracted, focused, polarized, and made to interfere. In a nutshell, he showed that electromagnetic waves and light waves were actually the same!
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