Generating And Detecting Electromagnetic Waves

Type of physical science: Classical physics

Field of study: Electromagnetism

Electromagnetic waves can be generated and detected by many different techniques depending on their wavelength. Applications include radio, television, radar, photography, medicine, and astronomy.

Overview

Electromagnetic waves in space can be generated and detected from oscillating electric charges. Such waves were first described and identified with light by James Clerk Maxwell.

Using the field concept of Michael Faraday, he was able to provide a mathematical formulation of the basic laws of electricity and magnetism. Faraday had described the magnetic effects of electric current, as observed by Andre-Marie Ampere in 1820, in terms of a magnetic field surrounding the current. In 1831, Faraday discovered electromagnetic induction, in which a changing magnetic field produces an electric voltage. Maxwell generalized these results in 1864, showing that a changing electric field behaves like a current in producing a magnetic field (Ampere's law), and a changing magnetic field generates an electric field (Faraday's law).

Maxwell combined the symmetric relationships between electric and magnetic fields in 1872, showing the possibility of their propagation in space as an electromagnetic wave. The speed of the wave could be calculated from electric and magnetic constants that describe the strength of the fields in Ampere's and Faraday's laws, giving a result that closely matched the measured speed of light. This strongly suggested that light itself is an electromagnetic wave.

Maxwell's equations showed that accelerating or oscillating electric charges generate electromagnetic waves that may differ from light in frequency and wavelength, but the product of these charges is always equal to the speed of light. In principle, these waves can be detected by the influence of their fields on electric charges and currents.

In 1887, Heinrich Hertz used Maxwell's theory to discover radio waves and to demonstrate their properties. He was able to generate radio waves from an oscillating spark discharge produced by an induction coil at a frequency of about 50 megahertz (50 million oscillations per second). His detector was a loop of wire with a small open gap in which weaker sparks appeared simultaneously with the generating spark. In moving the detector away from the transmitter, the spark alternated in strength at increasing distances corresponding to the minimum and maximum points on the radio wave. From the distances between these points, he measured a wavelength of about 6 meters. The product of frequency and wavelength matched the speed of light (50 megahertz x 6 meters = 300 million meters per second), confirming the existence of electromagnetic waves as predicted by Maxwell, but at a much lower frequency and longer wavelength than visible light.

The generation and detection of radio waves was rapidly developed by Sir Oliver Joseph Lodge, Guglielmo Marconi, and others. Lodge improved the detector in 1889 by placing the spark gap in a glass tube connected to a battery and galvanometer to measure current. This device, called a coherer, could respond to an electric field too weak to produce a spark but strong enough to remove electrons from air in the tube (ionization) and to permit current to flow.

Marconi developed an improved transmitter and longer wire antennas to strengthen radiated fields and the currents they induce in the receiving antenna. He was able to transmit radio waves across the Atlantic Ocean by the end of 1901. More effective transmitters and receivers were developed in the twentieth century with the invention of vacuum tubes and eventually transistors for improved oscillators and detectors.

Maxwell's electromagnetic theory, as confirmed by Hertz, opened up a new understanding of the whole spectrum of electromagnetic waves. In 1800, Sir William Herschel had explored temperature variations across the colors of the visible spectrum produced by sunlight passing through a prism. He discovered infrared radiation at wavelengths longer than the longest waves of visible light by observing the higher temperatures found beyond the red end of the visible spectrum. In 1801, Johann Wilhelm Ritter found that chemical reactions produced by light were even stronger beyond the violet end of the spectrum, revealing the ultraviolet radiation at wavelengths shorter than visible light. These chemical reactions led to the development of photography as one of the most important techniques for detecting ultraviolet, visible, and infrared radiation. Wavelengths were measured by passing radiation through closely spaced slits (diffraction grating) and observing where waves were reinforced by interference effects. Infrared was found to have wavelengths up to about one hundred times longer than light waves, and ultraviolet waves were up to one hundred times shorter than visible light.

Electromagnetic waves even shorter than ultraviolet were first discovered by Wilhelm Conrad Rontgen in 1896. They were generated from a high-voltage vacuum tube (cathode-ray tube) in which high-energy electrons from a negative electrode (cathode) bombarded the positive electrode (anode). This produced a glow in the glass from radiation produced by these electron collisions, which Rontgen called X rays. He also showed their penetrating effects on a photographic plate. In 1913, Lawrence Bragg measured their wavelengths by reflecting X rays from a crystal and found them to be up to more than one hundred times shorter than ultraviolet waves. After the discovery of radioactivity in 1896, two of its components were found to be deflected oppositely by a magnetic field and were designated as α and β "radiation" (later shown to be helium nuclei and electrons). A third component called γ rays were undeflected by magnetism and identified as electromagnetic waves even shorter than X-ray wavelengths.

In general, electromagnetic waves are generated by oscillating charges and detected from the effects of their electric and magnetic fields. With the development of atomic theory by Niels Bohr in 1913, the emission and absorption of electromagnetic waves by atoms were identified with electron transitions between energy levels (orbits) within the atom or its nucleus.

Such transitions generate discrete frequencies producing well-defined lines in the spectrum.

Oscillating charges, resulting from thermal or other sources of energy, usually generate a range of frequencies that produce a continuous spectrum.

Radio waves are generated in nature by lightning discharges and other charge disturbances. Controlled radio waves are generated by electronic oscillators feeding wire antennas. They are detected by shorter antennas connected to electronic circuits, whose elements can be adjusted for maximum response (resonance) at a particular frequency, and the resulting currents can be rectified into direct currents with a diode for producing sound. With the development of radar during World War II, methods were developed for generating centimeter radio waves called microwaves. High-frequency oscillators such as klystrons and magnetrons were developed, and the resulting microwaves were focused by reflectors or horns to produce directional beams.

Infrared radiation at wavelengths shorter than a millimeter is generated from thermal and elastic vibrations of molecules. Infrared can be detected from its absorption by molecules and thermal effects associated with their increased vibrations. Visible and ultraviolet waves are generated by electron transitions in atoms, which are produced by high temperatures or in an electron discharge tube, and can be detected by optical, electrical, and chemical effects. These include photography, fluorescence, and the photoelectric effect, a phenomenon in which electrons are excited in a photocell by absorbed radiation.

X rays are generated by the deceleration of high-energy electrons in a cathode-ray tube as they penetrate the anode and as they eject inner atomic electrons causing transitions between inner orbits. Gamma rays are generated by nuclear transitions resulting from radioactivity or induced by neutron absorption or particle acclerators. X rays and γ rays can be detected by photographic techniques and from the ionization they produce in devices such as Geiger counters, ionization chambers, and scintillation counters, in which fluorescent materials emit flashes of visible light.

Applications

The principles discussed in the "Overview" section can be illustrated by a number of examples and applications. One example that involves generation and detection of microwaves is radar, an acronym for radio detection and ranging. In a radar set, pulsed microwaves are generated by a magnetron, wherein electrons from a heated cathode are accelerated in a series of resonant cavities by electric and magnetic fields to yield centimeter waves. The microwaves are then fed through a waveguide (metallic tube) to the antenna, consisting of a reflector or an array of radiators that focus the waves into a narrow beam and allow it to be pointed in different directions (scanned) to illuminate a target. The returned reflections are received by the same antenna, amplified, and displayed on an indicator such as a cathode-ray tube. Direction and range of the target are indicated from the direction of the antenna and the echo time between the transmitted and received pulse.

Both microwaves and infrared radiation can be detected by thermal effects. A thermocouple consists of two dissimilar conductors joined at their ends. When one end is heated by radiation, a thermoelectric voltage is established between the two junctions proportional to the temperature differences. Sensitivity of a thermocouple can be increased by placing it in a vacuum tube, with a thermopile formed from an array of thermocouples. A bolometer is a device that measures the energy of radiation from the increase in electrical resistance that results when a blackened metal foil is heated by absorbing radiation.

Infrared, visible, and ultraviolet radiation can be detected by the photoelectric effect, first observed by Hertz in 1887. In a photo tube, radiation absorbed by the cathode releases electrons with energy proportional to the frequency of the radiation. Semiconductor crystals have been developed that have the ability to absorb individual photons (quantum units of radiation) and release an electron unit of charge with a proportional change in the conductivity of the crystal. Several such photoconductive detectors can be arrayed to form a charge-coupled device (CCD) so that the electric charge at the output of one provides the input stimulus to the next. A matrix of such arrays is used in a television camera for changing light images into electrical signals.

Controlled radiation can be generated over a wide range of frequencies by masers and lasers. Both of these devices use the concept of stimulated emission of radiation by atoms, first described by Albert Einstein in 1916. This is a process in which an atom in an excited energy state can be stimulated to emit a photon by an incident photon, both with identical frequencies proportional to the transition energy of the atom. The first maser was developed in 1954 at Columbia University, and the first laser (acronyms for microwave or light amplification by the stimulated emission of radiation) was invented by Theodore H. Maiman in 1960. Both devices use materials in a resonant cavity, crystal, or gas discharge tube, whose atoms can absorb energy and remain in an excited state until emission is stimulated by photons with the same frequency as emitted photons. Amplification results from the cascading effect, in which each emitted photon can cause further emissions. The resulting radiation is very close to a single frequency (monochromatic) and the waves are all in step with one another (coherent). A narrow unidirectional beam can be produced by parallel mirrors at each end of the active material so that emitted radiation perpendicular to the mirrors will be amplified as a result of multiple reflections.

One mirror is partially silvered to allow some fraction of the beam to escape.

The generation of X rays can be controlled by varying the accelerating voltage in a cathode-ray tube and by the use of different target metals on the anode. Increasing the voltage increases the energy of the electrons and the frequency of the resulting X rays when the electrons strike the anode. Frequencies of discrete radiation from electron transitions also increase for target elements with larger atomic numbers. In a similar way, γ radiation can be controlled by the energy of a particle accelerator such as a cyclotron and by using different targets to produce different nuclear reactions.

Perhaps the most extensive use of detectors for electromagnetic radiation is in the field of astronomy. Most large optical telescopes collect light in a concave mirror, or by multiple reflectors, and bring it to a focus at an eyepiece lens or photographic film. Infrared and ultraviolet waves can be focused in a similar way on photographic film. Photoelectric detectors, such as photocells and CCDs, are more sensitive and can be used over a wider range from infrared through X rays. The first radio telescope, constructed by Grote Reber in 1937, consisted of a 10-meter dish that focused radio waves into a resonant cavity containing a dipole antenna split at the center to feed current into an amplifier and ammeter. Radio telescopes up to 300 meters in diameter have been constructed to resolve the longer wavelengths of radio waves.

Context

The generation and detection of electromagnetic waves are important in many areas of modern science and technology. Perhaps the most obvious area is in the field of radio communications. The invention of the diode vacuum tube by Sir John Ambrose Fleming in 1904 made it possible to detect high-frequency signals by changing them into direct currents that could drive a transducer such as a loudspeaker. The triode vacuum tube, invented by Lee de Forest in 1906, could be used to amplify weak signals. When the output of an amplifier was fed back through a resonant circuit to the input of the amplifier, it reinforced oscillations at the resonant frequency. The output amplitude of such a feedback oscillator could be further controlled with the signal from a microphone to generate amplitude modulated (AM) radio waves for transmitting sound information.

Standard AM radio is broadcast on radio waves in the range of 550 to 1600 kilohertz, while frequency modulated (FM) radio uses frequency variations to carry information in the range of 88 to 108 megahertz. Neighboring frequencies are used to transmit television, and other frequencies are assigned for a variety of communication purposes. The invention of the transistor amplifier in 1948 was followed by the development of solid-state integrated circuits, leading to significant miniaturization and improvement of radio transmitters and receivers. Many radio, telephone, and television signals can be transmitted on a single microwave beam, and orbiting communication satellites relay these signals around the earth.

Electromagnetic waves have also become important in the field of medicine. The discovery of X rays led very quickly to photographs of skeletal features for diagnosing bone fractures. Further development of X-ray techniques has made possible the diagnostic imaging of many internal organs. The identification of γ radiation led to the discovery of its effect on tumors and the use of radioactive sources for cancer therapy. Injection of such sources can be used to diagnose various internal body organs by detecting the spread of radiation. Infrared and microwaves are used in diathermy for heating internal body tissues. Infrared imaging is used in thermography to detect temperature variations caused by tumors or other problems. Perhaps the most dramatic applications have been developed from various types of laser sources. These include laser eye surgery to "weld" detached retinas and vaporize growths, plaque removal in blood vessels, and self-cauterizing incisions. Fiber optics can be used to transmit light into various parts of the body for surgical and diagnostic purposes.

The detection of radiation over the entire electromagnetic spectrum has led to many dramatic discoveries in astronomy and an expanding conception of the universe. The development of spectroscopy and photography in the nineteenth century and large optical telescopes in the twentieth century led to the discovery of stellar and galactic structure and the expansion of the universe. X-ray astronomy has produced the best evidence for the existence of black holes in space from the generation of X rays when matter is accelerated from a star to a massive binary partner. Radio astronomy has revealed the spiral-arm structure of the Milky Way galaxy, the existence of rotating neutron stars known as pulsars, and microwave background radiation that confirmed the big bang origin of the expanding universe.

Principal terms

ANTENNA: a device used for radiating and receiving radio waves

ELECTRIC AND MAGNETIC FIELDS: the influences permeating space caused by electric charges; electric fields transmit attraction and repulsion of charges, and magnetic fields exist around and interact with moving charges (currents)

ELECTROMAGNETIC SPECTRUM: the total range of electromagnetic waves arranged according to decreasing wavelengths from radio waves to microwaves, infrared radiation, visible light, ultraviolet, X rays, and γ rays

ELECTROMAGNETIC WAVES: waves that consist of electric and magnetic fields oscillating at right angles to each other and to their direction of motion at the speed of light

ELECTRIC OSCILLATOR: an electronic circuit or resonant structure that produces an alternating electric output at a frequency (cycles per second or hertz) determined by its electrical properties

RESONANCE: maximum response (amplitude of oscillation) of an oscillating system at the frequency of the natural free oscillations of the system

TRANSDUCER: a device, such as a microphone, loudspeaker, or photocell, to convert an input into an output of a different form

Bibliography

Bertolotti, M. MASERS AND LASERS: AN HISTORICAL APPROACH. Bristol, England: Adam Hilger, 1983. A good historical survey of the theory and development of the most important types of masers and lasers, including graphs, diagrams, and historical photographs. Each chapter is well documented with many historical sources.

Davidovits, Paul. COMMUNICATION. New York: Holt, Rinehart and Winston, 1972. A very readable historical introduction to the basic ideas of electricity and magnetism and their applications to radio, television, microwave communications, radar, and lasers. Contains many excellent diagrams and interesting photographs. Concludes with forty-five references for further reading and an index.

Hewish, Antony, ed. SEEING BEYOND THE VISIBLE. New York: Elsevier, 1970. The seven essays have been supplemented by many good diagrams. The chapters discuss light, radio waves, X rays, and microwaves with applications that include astronomy, microscopy, radar, and lasers.

Ronan, Colin. INVISIBLE ASTRONOMY. Philadelphia: J. B.

Lippincott, 1972. A readable discussion of the electromagnetic spectrum and methods of observation for visual, radio, and ultraviolet astronomy. Some diagrams and black-and-white photographs are included with a brief bibliography and index.

Schawlow, Arthur L., ed. LASERS AND LIGHT. San Francisco: W. H.

Freeman, 1969. A collection of thirty-two SCIENTIFIC AMERICAN articles by experts in their fields of research on light, vision, X rays, infrared, radio waves, and lasers. Lavishly illustrated with diagrams and photographs.

Radio and Television

X-Ray and Gamma-Ray Astronomy