The discovery of electromagnetic wave

INTRODUCTION…Electromagnetic waves are formed when an electric field (shown as blue arrows) couples with a magnetic field (shown as red arrows). The magnetic and electric fields of an electromagnetic wave are perpendicular to each other and to the direction of the wave. Maxwell and Hertz are two scientists who studied how electromagnetic waves are formed and how fast they travel.

In Faraday's time the speed of light was known, although not as accurately as it is today. It was first derived around 1676 by Olaus Roemer, a Danish astronomer working in Paris. But what was the meaning of the link between electricity and light ???

Remember the idea of Faraday which evolved into the "magnetic field" concept--that space in which magnetic forces may be observed is somehow changed? Faraday also showed that a magnetic field could drive electric currents, if copper wires were placed in it in the appropriate way. That was "magnetic induction,“ .So, magnetic fields could produce electric currents, and we already know that electric currents produce magnetic fields. Would it perhaps be possible for space to support a wave motion alternating between the two? 

magnetic field ---> electric current ---> magnetic field ---> electric current ---> ...

Such a wave could not exist in empty space, because empty space contained no copper wires and could not carry the currents needed to complete the above cycle. James Clerk Maxwell, solved the riddle in 1861 by representing an electric current which could travel through empty space, but only for very fast oscillations.

Maxwell proposed that it indeed was light. For instance, if a beam of light hits the side of a glass prism, only part of it enters--another part gets reflected. Maxwell's theory correctly predicted properties of the reflected beam.

The drawing below illustrates such a wave--green is the magnetic part, blue the electric part--the term Maxwell added. The wave is drawn propagating just along one line.

Then Heinrich Hertz in Germany showed that an electric current bouncing back and forth in a wire (nowadays it would be called an "antenna") could be the source of such waves. Electric sparks create back-and-forth currents when they jump across a gap and Hertz in 1886 used such sparks to send a radio signal across his lab. Also, It was presumed that light from the hot wire of a light bulb was emitted because the heat caused electrons to bounce back and forth rapidly, turning each into a tiny antenna.

Between 1885 and 1889, he produced electromagnetic waves in the laboratory and measured their wavelength and velocity. He showed that the nature of their reflection and refraction was the same as those of light, confirming that light waves are electromagnetic radiation.Early experimental Hertz radiator and  resonator for creating and detecting Hertzian waves ~1890Simple spark gap apparatus similar to this was the first ever built to produce and detect radio waves

In 1895, the German physician Röntgen studied an electric current passing through a bulb in which the air was at low-pressure. He realized that the bulb produced rays capable of penetrating boxes, bags, and even the human body. He named them X-rays: Rays because they occur in straight lines like light, and X because he did not know their nature. Despite this doubt, X-rays were immediately used in medicine for radiography, but remained mysterious because no optical instrument was capable of reflecting them, focusing them, dispersing them, or even diffracting them.

It was only in 1912 that the German physician Max von Laue experimentally succeeded in obtaining the diffraction of X-rays with a crystal. They are electromagnetic waves, but with extremely short wavelengths, located beyond the ultraviolet spectrum. 

Gradually other electromagnetic waves were found .The wave nature of light causes different colors to be reflected differently by a surface ruled in fine parallel scratches--which is why a compact laser disk shimmers in all colors of the rainbow. 

 Electromagnetic waves led to radio and television, and to a huge electronic industry. But they are also generated in space--by unstable electron beams in the magnetosphere, as well as at the Sun and in the far-away universe, telling us about energetic particles in distant space, or else teasing us with unresolved mysteries

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