Electromagnetic Waves: Origin and Theory

Origin: -

The phenomena associated with electricity and magnetism was studied over most of the nineteenth century. But the knowledge that the two fields were interdependent began with the fantastic discovery by Hans Christian Orsted in the early 1820’s. He learnt that magnetism is ultimately caused by moving electric charges or current, when he observed a magnetic compass needle to react to a current flowing through a wire placed near it.

Later on the simultaneous though separate discoveries made by Michael Faraday and Joseph Henry concerning electromagnetic induction in the 1830’s led to the theory of James Clerk Maxwell, which united electricity, magnetism and optics into one grand theory of light : the explanation of electromagnetic waves.

Maxwell published his work Treastise on Electricity and Magnetism (1873), in which he showed that four fundamental mathematical equations described the entire known electric and magnetic phenomenon. The first equation is Gauss’s law for electricity, which states that positive and negative charges create magnetic fields; Gauss’s law for magnetism states that currents create magnetic field, which have associated north and south poles, but single poles (monopoles) do not exist; Ampere’s law states that time varying magnetic fields induce time varying electric fields; and faraday’s law of induction states that time varying electric fields create time varying magnetic fields. Additionally, Maxwell’s equation predicted the existence of combined, changing electric and magnetic fields in the form of waves that traveled with the speed of light i.e. electromagnetic waves. He speculated that accelerated charges ultimately create these electromagnetic waves, that they should exist over a wide range of frequencies and wavelengths, that they traveled at the speed of light in a vacuum, and that they exhibited all the optical properties of visible light, such as reflection, refraction and diffraction.

Heinrich Rudolf Hertz in 1887 verified Maxwell’s theory experimentally ten years after his death. Hertz built an induction coil device, which was essentially a step up transformer whose high output voltage caused, sparks to jump back and forth across an air gap between two metal plates. One wire, bent so that it too had an air gap between its ends, was placed near another wire. Hertz noticed sparks jumping across the ends of this wire at the same frequency as the induction coil’s sparks. He concluded that electromagnetic waves propagated through air from the coil to the bent wire. These waves proved to be radio waves of about 1 meter in wavelength. He demonstrated that the waves exhibited all the usual properties of light; namely, they reflected, focused on parabolic mirrors, and refracted through glass. He caused them to interfere, setting up a standing wave pattern that enabled him to calculate their speed to be the speed of light. Later experiments demonstrated that a wide range of electromagnetic wavelengths and frequencies exist and led to the technologies of radio, television, radar and myriad other technologies important to society.

Theory: -

Many natural phenomena exhibit wavelike behavior. Water waves, earthquake waves, and sound waves all require a medium or substance through which to propagate. These are examples of mechanical waves. Light can also be described as waves- waves of changing electric and magnetic fields that propagate outward from their sources. These electromagnetic waves however do not require a medium. They propagate at 3,000,000,00 meters per second through vacuum. Electromagnetic waves are transverse waves. In simpler terms, the changing electric and magnetic fields oscillate perpendicular to each other and to the direction of the propagating waves.

The best source of electromagnetic waves is accelerated waves. An accelerated charge is one that is increasing or decreasing its speed or changing its direction of motion or both. Let us imagine two charges at rest in the vicinity of each other. They are immersed in each others electric force field. If one charge suddenly begins to oscillate up and down, the second charge experiences the change in the field of the first charge after some very small finite time elapses. The oscillating charge was accelerated. The moving charge’s electric fields change, as do their magnetic fields. These changing electric and magnetic fields generate each other through Faraday’s law of induction and Ampere’s law. These changing fields dissociate from the oscillating charge and propagate out into space at the speed of light.

All periodic waves, whether they are electromagnetic or mechanical, are characterized by such properties as wave length, frequency, and speed. For electromagnetic waves, wavelength measures the distance between the successive pulses of electric or magnetic fields. A waves’ frequency represents how many wave pulses pass by a given point each second and is measured in cycles per second or waves per second and is measured in cycles per second or waves per second. One wave per second is called one Hertz. Electromagnetic waves travel at the speed of light in vacuum, but they travel more slowly when they pass through various media such as air, glass, and water. A relationship among frequency, wavelength and speed exists for electromagnetic waves; the product of frequency and wavelength equals the speed of light. Thus, wavelength and frequency are inversely related. The longer the frequency lower is the wavelength and vice versa.

An entire spectrum of electromagnetic waves exists, which ranges from very low frequency wavelength (power waves) to very high wavelength (gamma rays). All wavelengths are collectively referred to as electromagnetic wavelengths and not merely the narrow range of wavelengths and frequencies identified as visible light.

The wave nature of light describes many aspects of its behavior. Nevertheless, radiation also has its particle like characteristics. Rather than infinite or nearly infinite series of electromagnetic waves emanating from some accelerated charge, light also appears to come in particle –like bursts of energy. These individual bursts of energy or quanta are called photons. Each photon possesses an amount of energy that directly depends on the frequency of the associated electromagnetic wave. Doubling the frequency of the photon of radiation doubles its energy. Thus, all types of electromagnetic waves, photons of power waves possess the least energy and gamma-ray photons possess the greatest energy.

Since life on earth is bathed constantly in all forms of electromagnetic radiation, scientists must be aware of the potential risks, as well as benefits of exposures to electromagnetic waves.

References:-

1) Gamow ,George : The great Physicists from Galileo to Einstein
2) Mullingan, Joseph F. " Heinrich Hertz and the development of Physics", Physics Today 42(March 1989)
3) Olenick, Richard P. ,Tom M. Apostol, and David L.Goodstein. : Beyond the Mechanical Universe.