ELECTROMAGNETIC WAVES

Faraday’s law of E.M.I. suggests that a magnetic field changing with time at a point in space produces an electric field at that point. On the other hand, Maxwell’s concept of displacement current led to the conclusion that an electric field changing with time at a point in space produces an magnetic field at that point. The electric and magnetic field can propagate through space with velocity of light and were called E.M. waves.

The transverse time varying electric and magnetic fields propagating in space in a direction perpendicular to the direction of both the electric and magnetic fields are said to constitute ELECTROMAGNETIC WAVES.

PROPERTIES OF E.M. WAVES:

1. E.M. Waves are transverse in nature.2. They are produced by accelerated charges particles.3. In free space they travel with a velocity, c= ( µ0€0)-1/2 ( where µ0 and €0 are absolute

permeability and absolute permittivity of free space. In a material medium they travel with a velocity , v= (µ€)-1/2 ( where µ and € are permeability and permittivity of that medium.

4. They obey the principle of superposition.5. The variation in the amplitudes of electric and magnetic fields in e.m. waves take

place at the same time and at the same place in space. The ratio of electric and magnetic fields is always constant and is equal to the velocity of e.m. waves in that medium.

6. The energy in e.m. waves is divided equally between the electric and magnetic fields.

SOUCRCE OF E.M. WAVES :

An electric charge at rest has electric field in the region around it, but no magnetic field. A moving charge produces both electric and magnetic fields. If the current is constant the magnetic and electric fields will not change with time, hence no e.m. waves can be produced. If the charge is accelerated both magnetic and electric fields will change with space and time, it then produces electromagnetic waves. This shows that an accelerated charge emits electromagnetic waves.

HISTORY OF E.M. WAVES:

In 1865, Maxwell predicted the existence of e.m. waves. According to him an accelerated charge produces a time varying magnetic field which in turn produces a time varying electric field. The two fields so produced are mutually perpendicular to each other and are source of each other.

The mutually perpendicular time varying electric and magnetic fields constitute e.m. waves abd propagate in space in a direction perpendicular to both the direction of varying electric and magnetic fields.

In 1887, Hertz experimentally demonstrated the production of e.m. waves of wavelength 6m by using a spark oscillator and then detecting them successfully.

Seven year after in 1895 Jagdish Chander Bose produced e.m. waves of wavelength range 5mm to 25 mm.

In 1896 Marconi discovered that if one end of the spark gap is connected to an antenna and the other terminal is earthed the e.m. waves radiated upto few kilometres (He demonstrated the transmission of e.m. waves)

Electromagnetic Spectrum

The whole range of frequencies/wavelengths of the electromagnetic waves arranged in an order is known as the electromagnetic spectrum.

Electromagnetic spectrum consists of the following waves :

1. Radio waves 2. Micro waves 3. Infrared (i.e. IR) rays 4 Visible light

5. Ultra violet (i.e. UV) rays, 6. X-rays and 7. Gamma (i.e. ϒ ) rays

1. Radio waves : Radio waves are produced by rapidly changing the electric current in LC circuit. The wavelength of a radio wave is the longest among all the electromagnetic waves, whereas the frequency of radio wave is the least among all the electromagnetic waves. Wavelength range of radio wave is from 15 cm to 2000 m.

Uses. Radio waves are mainly used for communication purpose. Radio waves are classified into many categories depending upon their frequency range. The classification of radio waves and their uses are given below:

(i) Very low frequency (VLF) radio waves : The frequency range of very low frequency radio waves is 10 kHz to 30 kHz. This type of radio waves is used for point to point short distance communication.

(ii) Low frequency (LF) radio waves : The frequency range of the low frequency waves is 30 kHz to 300 kHz. This type of radio waves is used for marine communication and navigation.

(iii) Medium frequency (MF) radio waves : The frequency range of the medium frequency radio waves is 300 kHz to 3000 kHz. These radio waves are used for amplitude modulated AM broadcasting.

(iv) High Frequency (HF) radio waves : The frequency of the high frequency radio waves is 3 MHz to 50 MHz. These radio waves are used for long distance communication.

(v) Very high frequency (VHF) radio waves: the frequency range of VHF radio waves is 50 MHz to 300 MHz. These radio waves used for FM transmission, radar and telecasting television programmes.

(vi) Ultra high frequency (UHF) radio waves : The frequency of UHF radio waves is 300 MHz to 3000 MHz. These radio waves are used for long distance communication. Cellular phone make use UHF radio. Now, TV’s also making use of UHF, hyper waves and super bands upto 900 MHZ.

2. Microwaves . The wave kength of microwaves is greater than 1.0 mm and less than 30 cm. The frequency range of microwaves is 109 Hz to 3.0 X 10 11 Hz . They are produced by oscillating electrons in a cavity. The commonly used oscillators to produce microwaves are Klystron, Magnetron and Gunn diodes.

Uses:

(i) Microwaves are used in a radar communication

(ii) These are used for atomic and molecular research

(iii) These are used for aircraft navigation

(iv) These are used in microwave ovens for cooking and warming foods. Frequency of the microwaves is set around 3GHz which matches the resonant frequency of water molecules which make them to vibrate at larger amplitudes to produceheat. This heat is produced for cooking/warming food.

(v) Microwaves are used for communication by cellular phones.

(vi) Microwaves are used in weather radar.

3. Infra-red (IR) rays . The wave length range of infra-red rays is 1 mm to 700 nm and the frequency range is 3.0 X 1011 Hz to 4.3 X10 14Hz. Infra-red rays are produced by the excitation of atoms and molecules. Hot bodies also radiate infra-red rays. These are called heat waves also.

Uses :

(i) These rays can be pass through the haze, fog and mist, so these rays are used in night vision devices during warfare and for taking photographs of earth under foggy conditions from great height. Infrared detectors are used by earth satellites.

9ii) These rays are used to function the green houses because green house gas CO2 and water vapours absorb the infrared rays readily.

(iii) They are used in revealing the secret writings on the ancient walls.

(iv) Infrared lamps are used to treat mascular strains.

(v) The infra-red rays from the sun keep he earth warm.

(vi) Infrared remotes are used to operate electronic devises like TV, VCD, music hi-fi systems etc.

4. Visible light. The wavelength rane of visible* light is 400 nm (violet) to 780 nm (red) and the frequency range is 4.3 X 10 14 Hz to 7.5 X 10 14 Hz. Visible light is emitted when an electron jumps from higher energy level to lower energy level of an atom.

Uses :

(i) Visible light stimulates the sense of the sight in human beings, so the beautiful world around us is seen in the presence of visible light.

(ii) Visible light is useful in photography.

(iii) It is useful in optical microscopy.

(iv) It is useful in astronomy.

(v) It is a great source of energy for human life.

5. Ultra-violet (UV) rays: The range of ultra-violet rays is 400 nm to 0.6 nm and their frequency range is 7.5 X 10 14 Hz to 5.0 X 10 15 Hz. The sun is the most important source of ultra violet rays. Ultra-violet rays are also produced by the welding arc. These rays in large quantities are very harmful to the living tissues. They cause tanning of the skin due to their promotion of melanin in the skin. Prolonged exposure to Uv rays can induce cancer in human beings. Tanning can be protected by using glass panes because glass can absorb UV rays. Ozone layer also protects us from the UV radiations present in the sun rays.

Caution: The sun should not be directly looked at even during total solar eclipse. Ultra-violet rays constantly entering our eyes may cause total blindness.

Uses:

(i) They are used to preserve food stuff and make drinking water free from bacterias as these rays can kill bacteria, germs etc.

(ii) They are used for stirlizing the surgical instruments.

(iii) They are used in detecting the invisible writings, forged documents and finger prints.

(iv) They are used to study the structure of molecules.

(v) UV rays are used in LASIK eye surgery.;

6. X –rays . These rays were discovered by Prof. Rontgen in 1895. X-rays can be produced by bombarding a target of high atomic number (Z) with a beam of fast moving electrons. The range of the wavelength of X-rays varies from 10 nm to 10 -4 nm. The frequency of these rays varies from 5.0 X 10 15 Hz to 3.0 X 10 18 Hz. X-rays can penetrate through the human flesh but bones or metallic materials block these rays. These can be damaging for living tissues.

Uses:

(i) These are used in medical diagnosis like locating the fracture in the bone, foreign materials like coin or bullet in the body.

(ii) These are used in radio therapy to cure skin diseases, cancer and tumors.

(iii) These are used in engineering for locating the faults, cracks and flaws in the finished metallic materials.

(iv) X-rays are used by detective agencies to detect gold, silver, diamonds and other contraband goods etc. concealed in bags or the body of a person.

7. Gamma rays. These rays are produced when an electron jumps from higher orbit to the lower orbit of an atom. Y rays also produced during radioactive decay of nuclei and nuclear reactions. The wavelength of y-rays is the shortest of all the electromagnetic waves. The range of the wavelength of these rays varies from 10 -10 m to 10 -14 m. On the other hand, the frequency of y-rays is the highest of all the electromagnetic waves. The range of the frequency of y-rays varies from 3.0 X 10 18 Hz to 3.0 X 10 22Hz. The penetration power of these rays is extremely high.

Uses :

(i) These are used for the treatment of cancer.

(ii) These are used to examine the thick materials for structural flaws.

(iii) These are used for food preservation

(iv) These are used to get valuable information about the structure of atomic nuclei.