History of Electromagnetic Waves.
Faraday from his experimental study of electromagnetic induction concluded that a magnetic field changing with time at a point produces an electric field at that point. Maxwell in 1865 from his theoretical study pointed out “there is a great symmetry in nature”, i.e. an electric field changing with time at a point produces a magnetic field there. It means a change in either field (electric or magnetic) with time produces the other field.
This idea led Maxwell to conclude that the variation in electric and magnetic field vectors perpendicular to each other leads to the production of electromagnetic disturbances in space. These disturbances have the properties of wave and can travel in space even without any material medium. These waves are called electromagnetic waves. According to Maxwell, the electromagnetic waves are those waves in which there are sinusoidal variation of electric and magnetic field vectors at right angles to each other as well as at right angles to the direction of wave propagation.
Both these fields vary with time and space and have the same frequency. Maxwell also found that the electromagnetic wave should travel in free space (or vacuum) also. Maxwell also concluded that electromagnetic wave is transverse in nature and light is electromagnetic wave. Examples of electromagnetic waves are radio waves, microwaves, infrared rays, light waves, ultraviolet rays, X-rays and γ-rays.
In 1888, Hertz confirmed experimentally the existence of electromagnetic waves. With the help of his experiment, Hertz produced electromagnetic waves of wavelength about 6 m. In 1894, an Indian Physicist Jagdish Chander Bose was able to produce electromagnetic waves of wavelength ~ 5 to 25 mm but his experiment was confined to laboratory only. In 1899, Guylielmo Marconi was the first to transmit electromagnetic waves up to a few kilometers and established a wireless communication across the English Channel, a
distance of about 50 km.
Production of Electromagnetic Waves.
We know that an electric charge at rest has electric field in the region around it, but no magnetic field. A moving charge produces both the electric and magnetic fields. If a charge is moving with a constant velocity (i.e. if current is not changing with time), the electric and magnetic fields will not change with time, hence no electromagnetic wave can be produced. But if the charge is moving with a non-zero acceleration (i.e. charge is accelerated) both the magnetic field and electric fields will change with space and time, it then produces electromagnetic wave. This shows that an accelerated charge emits electromagnetic waves.
In an atom, an electron while orbiting around the nucleus in a stable orbit, although accelerating, does not emit electromagnetic waves. Electromagnetic waves are emitted only when it falls from higher energy orbit to lower energy orbit. Electromagnetic waves (i.e. X-rays) are also produced when fast moving electrons are suddenly stopped by the metal target of high atomic number.
Important Facts About the Electromagnetic Waves.
1. The electromagnetic waves are produced by accelerated or oscillated charge.
2. These waves do not require any material medium for propagation.
3. These waves travel in free space with a speed 3 × 108 m/s (i.e. speed of light).
4. The sinusoidal variation in both electric and magnetic field vectors (E and B) occurs simultaneously. As a result, they attain the maxima and minima at the same place and at the same time.
5. The directions of variation of electric and magnetic field vectors are perpendicular to each other as well as perpendicular to the direction of propagation of waves. Therefore, electromagnetic waves are transverse in nature like light waves.
6. The velocity of electromagnetic waves depends entirely on the electric and magnetic properties of the medium in which these waves travel and is independent of the amplitude of the field vectors.
7. The velocity of electromagnetic waves in dielectric is less than 3 × 108 m/s.
8. The energy in electromagnetic waves is equally divided between electric and magnetic vectors.
9. The electric vector is responsible for the optical effects of an electromagnetic wave and is called the light vector.
10. The electromagnetic waves being uncharged are not deflected by electric and magnetic fields.
Electromagnetic Spectrum.
Maxwell in 1865 predicted electromagnetic waves from theoretical considerations and their existence was confirmed experimentally by Hertz in 1888. Hertz experiment was based on the fact that an oscillating electric charge radiates electromagnetic waves and these waves carry energy which is being supplied at the cost of kinetic energy of the oscillating charge. The detailed study revealed that the electromagnetic radiation is significant only if the distance to which the charge oscillates is comparable to the wavelength of radiation. After the experimental discovery of electromagnetic waves by Hertz, many other electromagnetic waves were discovered by different ways of excitation.
The orderly distribution of electromagnetic radiations according to their wavelength or frequency is called electromagnetic spectrum. The electromagnetic spectrum has much wider range with wavelength variation of ~10-14 m to 6 × 106 m. The whole electromagnetic spectrum has been classified into different parts or subparts in order of increasing wavelength, according to their type of excitation. There is overlapping in certain parts of the spectrum, showing that the corresponding radiations can be produced by two methods. It may be noted that the physical properties of electromagnetic waves are decided by their wavelengths and not by the method of their excitation.
Uses of Electromagnetic Spectrum.
The following are some of the uses of electromagnetic spectrum:
1. Radio and microwave radiations are used in radio and TV communication system.
2. Infra-red radiations are used:
a. In revealing the secret writings on the ancient walls.
b. In green houses to keep the plants warm.
c. In war fare, for looking through haze, fog or mist as these radiations can pass through them.
d. In electrotherapy for the heating of soft tissues.
3. Ultra-violet radiations are used in the detection of invisible writing, forged documents, finger prints in forensic laboratory and to preserve the food stuffs. Ultra-violet radiations are used in electrotherapy for the treatment of various skin conditions.
4. X-rays can pass through soft tissues but not through bones. This property of X-rays is used in medical diagnosis, after X-ray films are made.
5. Electromagnetic waves of suitable frequencies are used in medical science for the treatment of various diseases.
6. Super high frequency electromagnetic waves are used in radar and satellite
communication.
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