Sources of Electromagnetic Waves: Electromagnetic waves were scientifically discovered by  James Clerk Maxwell, a British scientist, about \(150\) years ago. Heinrich Hertz was a brilliant physicist and experimentalist from Germany who demonstrated that the electromagnetic waves predicted by James Clerk Maxwell exist. Hertz is also the man whose peers honoured him by attaching his name to the unit of frequency; a cycle per second is one hertz.

J.C. Bose produced Electromagnetic waves of wavelength ranging from \(5\,{\rm{ mm}}\) to \(25\,{\rm{ mm}}\) experimentally. G Marconi successfully transmitted the EM waves up to a few kilometres. This article will explain the characteristics and sources of different electromagnetic waves.

What is Electromagnetic Induction?

According to Faraday’s law of electromagnetic induction: a time-varying magnetic field produces an electric field that changes with time. According to Ampere-Maxwell law, a time-varying electric field produces a magnetic field that changes with time. We can conclude from the above two laws that either field changes with time, then another field changing with time is induced in the space.

Maxwell showed that these variations of electric and magnetic fields occur in mutually perpendicular directions. These waves are called electromagnetic waves and can travel in space even without any material medium. The EM waves are transverse. EM waves do not require any material medium for their propagation. The electric field vector is responsible for the optical effects of an EM wave and is called the light vector.

Learn About Electromagnetic Spectrum

Definition of Electromagnetic Waves

Electromagnetic waves consist of a sinusoidal variation of the electric and magnetic field at right angles to each other and right angles to the direction of propagation of waves.

EM Spectrum The whole orderly range of frequencies/wavelengths of the EM waves is known as the EM spectrum.

Characteristics of Electromagnetic Waves

1. In an electromagnetic wave, the electric and magnetic fields are at right angles to each other. Both are perpendicular to the direction of propagation, and therefore electromagnetic waves are transverse.

2. Plane Electromagnetic Wave: A plane wave that travels in one direction and whose properties are independent of position in the plane perpendicular to the propagation direction is called a plane electromagnetic wave. The wave travels in the \(x\)-direction, the electric field \(E\) in the y-direction, and the magnetic field \(B\) in the z-direction. The electric and magnetic fields vary with time \(t\) and \(x\) therefore, wave due to electric and magnetic field represented as
\(E = {E_0}\sin \left( {kx – \omega t} \right)\,{\rm{or}}\,E = {E_0}\cos \left( {kx – \omega t} \right)\) And
\(B = {B_0}\sin \left( {kx – \omega t} \right)\,{\rm{or}}\,B = {B_0}\cos \left( {kx – \omega t} \right)\) Where \({E_0}\) and \({B_0}\)are the maximum values of electric and magnetic field respectively.
\(k = \frac{{2\pi }}{\lambda },\) It is a constant,
Where,
\(\lambda  = \) wavelength,
\(\omega  = 2\pi f = \) angular frequency,
\(f = \)  frequency of the wave.
The ratio \(\frac{\omega }{k} = \frac{{2\pi f}}{{\frac{{2\pi }}{\lambda }}} = f\lambda = c = {\rm{speed}}\,{\rm{of}}\,{\rm{light}}\)

3. Speed: In space, its speed \(c = \frac{1}{{\sqrt {{\mu _0}{\varepsilon _0}} }} = \frac{{{E_0}}}{{{B_0}}} = 3 \times {10^8}{\rm{m/s}}.\)
In medium \(v = \frac{1}{{\sqrt {\mu \varepsilon } }}\) where \({\mu _0} = \) Absolute permeability, \({\varepsilon _0} = \) Absolute permittivity
\({E_0}\,{\rm{and}}\,{B_0} = \) Amplitudes of electric field vector and magnetic field vectors.

4. Energy: The Energy in an EM wave is equally divided between the electric and magnetic fields.
The energy density of electric field \({u_e} = \frac{1}{2}{\varepsilon _0}{E^2}\),
The energy density of the magnetic field \({u_B} = \frac{1}{2}\frac{{{B^2}}}{{{\mu _0}}}\)
It is found that \({u_e} = {u_B}.\) Also \({u_{av}} = {u_e} + {u_B} = 2{u_e} = 2{u_B}\) \(= {\varepsilon _0}{E^2} = \frac{{{B^2}}}{{{\mu _0}}}\)

5. Intensity (I): The Energy of the wave crossing per unit area per unit time, perpendicular to the EM wave’s propagation direction, is called intensity.
\(I = {u_{av}} \times c = \frac{1}{2}{\varepsilon _0}{E^2}c = \frac{1}{2}\frac{{{B^2}}}{{{\mu _0}}}.c\)

6. Momentum: EM waves also carry momentum; if a portion of the EM wave of Energy \(u\) propagating with speed \(c,\) then linear momentum \( = \frac{{{\rm{Energy (}}u{\rm{)}}}}{{{\rm{Speed (}}c{\rm{)}}}}\)

Note:

1. When a surface completely absorbs the Electromagnetic incident wave, it delivers energy \(u\) and momentum \(\frac{u}{c}\) to the surface.

2. When a wave of energy \(u\) is reflected from the surface, the momentum delivered to the surface is \(\frac{{2u}}{c}.\)

7. Poynting vector \(\left( {\vec S} \right)\) : In EM waves, the rate of flow of Energy crossing a unit area is described by the Poynting vector.  Its unit is \({\rm{watt/}}{{\rm{m}}^{\rm{2}}},\) and \(\vec S = \frac{1}{{{\mu _o}}}\left( {\vec E \times \vec B} \right) = {c^2}{\varepsilon _0}\left( {\overrightarrow E \times \overrightarrow B } \right)\) \(\overrightarrow E,\) \(\overrightarrow B \) are perpendicular to each other, the magnitude of \(\overrightarrow S \) is \(\left| {\vec S} \right| = \frac{1}{{{\mu _0}}}E\,B\,\sin {90^o} = \frac{{EB}}{{{\mu _0}}} = \frac{{{E^2}}}{{\mu \,C}}\)

Note:

1. The direction of the Poynting vector \(\overrightarrow S \) at any point gives the wave’s direction of travel and direction of energy transport.

8. Radiation pressure: It is the momentum imparted per second per unit area on which the light falls.
For a perfectly reflecting surface \({P_r} = \frac{{2S}}{c},\) \(S = \) Poynting vector; \(c = \) speed of light
For a perfectly absorbing surface \({P_a} = \frac{S}{c}.\)

Note:

1. The radiation pressure is real and practical. E.g., the tail of a comet points away from the Sun.

Electromagnetic Spectrum and Sources of Electromagnetic Waves

The electromagnetic waves have been produced or detected over a wide range of frequencies. These are generally classified according to frequency or wavelength. The orderly classification of electromagnetic waves according to their wavelength or frequency is called the electromagnetic spectrum.

Name	Wavelength	Frequency (in Hz)	Discoverer
Gamma rays	\(0.01\;A^\circ\) to \(0.1\;A^\circ \)	\(3 \times {10^{22}}\) to \(3 \times {10^{19}}\)	Paul Ulrich Villard
X-rays	\(0.1\;A^\circ\) to \(100\;A^\circ \)	\(3 \times {10^{19}}\) to \(5 \times {10^{17}}\)	Wilhelm Rontgen
UV-rays	\(100\;A^\circ\) to \(4000\;A^\circ \)	\(5 \times {10^{17}}\) to \(8 \times {10^{14}}\)	J.W Ritter
Visible rays	\(4000\;A^\circ\) to \(8000\;A^\circ \)	\(8 \times {10^{14}}\) to \(4 \times {10^{14}}\)	Unknown
IR-rays	\(8000\;A^\circ\) to \(4000\,{\rm{m}}\)	\(4 \times {10^{14}}\) to \({10^{11}}\)	William Herschel
Micro waves	\(4000\,{\rm{m}}\) to \({10^{ – 2}}{\rm{m}}\)	\({10^{11}}\) to \({10^9}\)	Experimentally generated by Heinrich hertz
Radio waves	\({10^{ – 2}}\,{\rm{m}}\) to \({10^5}\,{\rm{m}}\)	\({10^9}\) to few hertz	Experimentally generated by Heinrich hertz

Gamma \(\left( \gamma \right)\) Rays

These are the electromagnetic radiations of the shortest wavelength.   Their wavelength range is from \(0.01{A^ \circ }\,{\rm{to}}\,0.1{A^ \circ }\) and the frequency range from \(3 \times {10^{22}}\,{\rm{Hz}}\,\,{\rm{to}}\,\,3 \times {10^{19}}\,{\rm{Hz}}\)

Sources of Gamma rays: The sources of Gamma rays are radioactive decay, Cosmic rays.

Rutherford’s experiment proves that when a sample of a radioactive substance(Uranium salt) is put in a lead box and allowed to emit radiations through a small hole only, three kinds of radiations are emitted-

1. Radiations that deflect towards the negative plate are called \(\alpha\)-rays.

2. Radiations that deflect towards positive plate are called \(\beta\) particles.

3. Radiations that are undeflected are called \(\gamma\)-rays.

Gamma rays are also emitted after \(\alpha \) decay or \(\beta \) decay as follows:
When a nucleus emits \(\alpha \) or \(\beta \) particle, the daughter nucleus may be left in an excited state of higher energy. In such cases, the daughter nucleus de-excites to the ground state by emitting one or more photons. Since the nuclear energy level spacings are of the order of MeV, the emitted photon is of an extremely short wavelength i.e., \(\gamma \) ray.

Fig – Gamma decay

Example:  Successive emission of \(\gamma \) rays of energy \(1.17{\rm{ MeV}}\) and \(1.33{\rm{ MeV}}\) from the deexcitation of \(_{28}{\rm{N}}{{\rm{i}}^{60}}\) nucleus formed from \(\beta – \) decay of \({_27}{\rm{C}}{{\rm{o}}^{60}}.\)

Properties of Gamma Rays

1. They affect photographic plates. 

2. They produce the photoelectric effect.

3. Gamma rays have a very high penetrating power.

4. They are dangerous to live tissues. 

5. They have low ionising power.

Uses of Gamma Rays

They are used:

1. To gauge the thickness of a metal sheet, paper, etc.

2. To kill the cancerous cells.

3. To detect the defect in metal casting.

4. The study of gamma rays provides information on the structure of atomic nuclei.

5. To initiate the nuclear reactions.

X-rays

Scientist Wilhelm Rontgen discovered x-rays. Hence, they are also called Rontgen rays. Their wavelength range is from \(0.1{A^ \circ }\,{\rm{to}}\,100{A^ \circ }\) and their frequency range is from \(3 \times {10^{19}}\,{\rm{Hz}}\,\,{\rm{to}}\,\,5 \times {10^{17}}\,{\rm{Hz}}.\)

Sources of X-rays

They are produced when a suitable metallic target suddenly stops fast-moving electrons.  They are produced using Rontgen’s gas-filled tube, Coolidge’s tube, and fast electrons stopped by the solid target.
Rontgen observed that when the pressure inside a discharge tube is kept at \({10^{–3}}\,{\rm{mm}}\) of \({\rm{Hg}}\) and voltage is \(25{\rm{ kV}},\) the anode emits some unknown radiations (X-rays). Since an unknown quantity is represented by X, these rays were called X-rays.

Coolidge X-ray Tube

There are three basic requirements for the production of X-rays. They are

(i) Electron source.

(ii) Arrangement to accelerate electrons.

(iii) A metal target of suitable material of high atomic mass and high melting point on which high-speed electrons strike.

Coolidge X-ray tube consists of a highly evacuated glass tube containing a cathode and a metal target. The cathode(negative terminal) consists of a tungsten filament. The filament(F) is coated with barium or strontium oxides to have electrons emitted even at low temperatures. The filament is surrounded by a cylinder made of molybdenum kept at negative potential w.r.t. the target.

The target (of high melting point, high atomic weight, and high thermal conductivity) made of molybdenum or tungsten is embedded in a block made of copper. The plane of the target is set at \({45^{\rm{o}}}\) to the incident electron stream.

Fig – Coolidge X-ray tube

The filament(F) is heated by passing a current through it. A high potential difference (\( \approx 10\) to \(80\;{\rm{ kV}}\)) is applied between the target and cathode to accelerate the electrons emitted by filament. The highly energetic electron streams are focused on the fixed target. \(98\%\) energy of the electrons is converted into heat, and \(2\%\) of the energy of the electrons produces \(X\)-rays.
A huge quantity of heat is produced in the target, conducted through the copper anode to the cooling fins from where it is dissipated by convection and radiation.

Note:

1. Production of \(X\)-ray is the reverse phenomenon of the photoelectric effect.

2. The Energy of bombarding electrons will be large for large potential difference, and hence larger is the penetration power of \(X\)-rays.

Properties of X-rays

1. They affect photographic plates. 

2. They produce the photoelectric effect. 

3. They cause fluorescence and phosphorescence.

4. They are penetrating and can pass through soft materials like flesh, thin metal sheets, wood, etc.

5. They have high ionising power.

Uses of X-rays

They are used

1. In the detection of dislocation and fractures of bones.

2. In the detection of foreign bodies like bullets, pins, coins, etc., in a human body.

3. In the determination of defects in castings, welding, rubber tyres, etc.

4. In the study of the structure of crystals.

5. To kill the tumours in the body.

UV-rays

Radiations lying beyond the violet light of the visible spectrum are called UV-rays.  Their wavelength range is from \(100{A^{\rm{o}}}\,{\rm{to}}\,\,4000{A^{\rm{o}}},\) and the frequency range is from \(5 \times {10^{17}}\,{\rm{Hz}}\,\,{\rm{to}}\,\,8 \times {10^{14}}\,{\rm{Hz}}.\)

Sources of UV-rays

Sun is the natural source of UV rays.  Artificial sources are mercury vapour lamps, discharge tubes, Arc lamps, etc.
UV radiation is produced by heating a body to an incandescent temperature, as in solar UV, or passing an electric current through vaporised mercury. The latter process is the mechanism whereby UV radiation is produced artificially. Both the quality (spectrum) and quantity (intensity) of terrestrial UV radiation vary with factors, including the elevation of the Sun above the horizon and absorption and scattering by molecules in the atmosphere, notably ozone and by clouds.

Fig – Mercury vapour lamp

Detectors of UV-rays: Photographic plate and photoelectric tube

Properties of UV-rays

1. They affect photographic plates. 

2. They produce the photoelectric effect. 

3. They cause fluorescence and phosphorescence.

4. They cause decomposition in organic acids.  

5. The ordinary glass absorbs them; therefore, in the study of UV radiations, quartz, fluorite prisms are used.

6. Glass is opaque to UV radiation; hence welders use protective goggles made of glass.

Uses of UV-rays

They are used:

1. In the production of Vitamin-D.

2. In the sterilization of water and air in hospitals.

3. In the treatment of diseases of bones like rickets.

4. In the high resolving power microscopes. 

5. In the activation of certain chemical reactions.

6. To distinguish between real and artificial gems.

7. To examine the fingerprint on the surface.

8. In the detection of forgeries in documents and adulteration of food.

Visible Light

The part of the EM spectrum to which the human eye is sensitive is visible light. Their wavelength ranges from \(4000{A^{\rm{o}}}\,\,{\rm{to}}\,\,8000{A^{\rm{o}}}.\) and the frequency is \(8 \times {10^{14}}\,{\rm{Hz}}\,\,{\rm{to}}\,\,4 \times {10^{14}}\,{\rm{Hz}}.\) The eye interprets this wavelength range as different colours.

Sources of Visible Light

Sun is the natural source of Visible light, and the electric bulb is the artificial source of visible light.

Fig – Electric bulb of different colours

Properties of Visible Light

1. They affect photographic plates.  

2. They produce the photoelectric effect.      

3. They cause fluorescence and phosphorescence. 

4. They produce chemical effects.

Uses of Visible light

Most of the information we get through our vision is possible because of visible light.  For our countless needs, we need light.

Infrared rays

Radiations lying beyond the red end of the visible spectrum are called IR-rays.   Their wavelength ranges from \(8000{A^{\rm{o}}}\,\,{\rm{to}}\,\,4000\,\mu {\rm{m}},\) frequency ranging from \(4 \times {10^{14}}\,{\rm{Hz}}\,\,{\rm{to}}\,\,{10^{11}}\,{\rm{Hz}}.\)

Sources of Infrared Rays

All hot bodies emit IR-rays, of which the Sun is a natural source. Nernst filament, pointolite lamp, Infrared LED, etc., are artificial sources.

Fig – Pointolite lamp and Nernst filament lamp

Detectors of Infrared Rays

1. Thermocouple and thermopile.

2. Bolometer and photo conducting cell.

Properties of Infrared Rays

1. They affect photographic plates.

2. They produce the photoelectric effect. 

3. They produce the sensation of heat when they fall on the skin.

4. They dilate the blood vessel.

5. They are less scattered compared to visible light, and they can pass through thick fog and smoke.

6. They undergo reflection, refraction, interference, polarisation.

Uses of Infrared Rays

They are used:

1. In long-distance photography.

2. In medicine, IR from a glowing filament heated electrically is used to treat muscle sprains and physiotherapy.

3. To activate blood circulation.

4. To produce the dehydrated fruits and also used in cooking and heating.

5. To analyse chemicals. 

6. To detect healthy crops by earth resource satellite.

Microwaves

These are the electromagnetic radiations with a wavelength range from \(4000\,\mu {\rm{m}}\,\,{\rm{to}}\,{10^{ – 2}}{\rm{m}}\) and a frequency range from \({10^{11}}\,{\rm{Hz}}\,\,{\rm{to}}\,\,{10^9}\,{\rm{Hz}}.\)

Sources of Microwaves

The sources of microwaves are the Klystron valve, Magnetron valve, and Gunn diode.
A Klystron valve or the magnetron valve are generally used to produce electromagnetic waves, and that electromagnetic wave is Microwave. Microwaves are also known as heating waves. They are used in microwave ovens for heating and cooking purposes. One distinct feature of Microwaves is that they are reflected by metal but can pass through water.
Gunn diode is an electronic device composed of only one type of semiconductor material, i.e., N-type. It utilises the negative resistance characteristics to generate current at high frequencies by the Gunn effect. J.B. Gunn discovered the Gunn effect in the early 1960s.

Fig – Klystron valve

Fig – Gunn diode

Uses of Microwaves

1. For radar communication and satellite communication.

2. In a microwave oven which reduces cooking time considerably.

3. To study the atomic and molecular structure.

Radio Waves

These are the electromagnetic radiations with wavelength ranges from \({10^{ – 2}}{\rm{m}}\,\,{\rm{to}}\,\,{10^5}{\rm{m}}\) and frequency ranges from \({10^9}{\rm{Hz}}\,\,{\rm{to}}\,{\rm{a}}\,{\rm{few}}\,{\rm{Hz}}.\)

Sources of Radio Waves

L-C oscillator is the best source of radio waves.

Heinrich Hertz’s experiment is used to produce Radiowaves based on the fact that an accelerating and oscillating charge will radiate electromagnetic waves(radio waves).

The following figure shows that the metallic plates (P1 and P2) act as a capacitor(source of EM waves). The wires connecting spheres S₁ and S₂ to the plates are used to provide a low inductance.

Fig – Production of Radio waves

When very high voltage is applied across metallic plates, the plates get discharged by sparking across the narrow gap. The spark will give rise to oscillations which in turn send out EM waves. The frequency of these waves is given by \(\nu = \frac{1}{{2\pi \sqrt {LC} }}.\)
The succession of sparks sends out a train of such waves, which are received by the receiver.

Types of Radiowaves

1. The frequency range from \(530{\rm{ kHz}}\) to \(1710{\rm{ kHz}}\) is for an amplitude-modulated band.   

2. The frequency range from \(1710{\rm{ kHz }}\) to \(54{\rm{ MHz}}\) is for a short wave band.

3. The frequency range from \(54{\rm{ MHz}}\) to \(890{\rm{ MHz}}\) is for television waves.

4. The frequency range from \(88{\rm{ MHz}}\) to \(108{\rm{ MHz}}\) is for frequency modulated band, used in commercial FM radio.

Uses of Radiowaves

They are used for radio, TV, navigation, and far communication purposes.

Summary

1. The whole orderly range of frequencies/wavelengths of the EM waves is known as the EM spectrum.
2. The sources of Gamma rays areRadioactive decay, Cosmic rays.
3. The sources of \(X\)-rays are Rontgen’s gas-filled tube, Coolidge’s tube, and fast electrons stopped by the solid target.
4. The sources of UV rays are Sun, mercury vapour lamps, discharge tubes, Arc lamps, etc.
5. The sources of Visible light are Sun, the electric bulb, etc.
6. The sources of Microwaves are Sun, Nernst filament, pointolite lamp, Infrared LED, etc.
7. The sources of microwaves are the Klystron valve, Magnetron valve, Gunn diode, etc.
8. The source of Radiowaves is the LC oscillator.

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FAQ’s on Sources of Electromagnetic waves

Q.1. What are the main sources of electromagnetic waves?
Ans: The main sources of electromagnetic waves are cosmos (e.g., the sun and stars), radioactive elements, and acceleration of charges.

Q.2. Which is the most important electromagnetic wave?
Ans: The most important electromagnetic wave is visible light because it enables us to see.

Q.3. What are the seven electromagnetic waves?
Ans: The seven electromagnetic waves are Gamma rays, \(X\)-rays, ultraviolet rays, visible light, infrared rays, microwaves, and radio waves.

Q.4. What is the source of UV rays?
Ans: Ultraviolet (UV) radiation is non-ionising radiation emitted by the Sun and artificial sources, such as tanning beds.

Q.5. What is the natural source of UV rays, visible rays, and Infrared rays?
Ans: Sun is the natural source of UV rays, visible rays, and Infrared rays.

Q.6. What is the source of a microwave wave?
Ans: Microwaves can be produced by the oscillation of atoms and molecules. Microwave sources include artificial devices such as circuits, transmission towers, radar, masers, and microwave ovens.

We hope this detailed article on the sources of electromagnetic waves helped you in your studies. If you have any doubts, queries or suggestions regarding this article, feel to ask us in the comment section and we will be more than happy to assist you. Happy learning!