Electromagnetic waves class 12 Notes, Ncert solutions, important questions

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Electromagnetic waves class 12 Notes : These waves are produced due to the change in electric field E and magnetic field B sinusoidally and propagating through space such that, the two fields are perpendicular to each other and perpendicular to the direction of wave propagation.

Source of Electromagnetic waves class 12 Notes

Maxwell found that the accelerated or oscillating charge radiates electromagnetic waves.

These charges produce an oscillating electric field in space, which produces an oscillating magnetic field, which in turn is a source of oscillating electric fields and so on. The oscillating electric and magnetic fields regenerate each other as a continuous wave that propagates through space. The frequency of the EM waves is equal to the frequency of oscillation of charge.
i.e.

Electromagnetic waves class 12 Notes
Electromagnetic Waves

Nature of Electromagnetic waves class 12 Notes

In an electromagnetic wave, electric and magnetic fields are perpendicular to each other and to the direction of wave propagation. A plane electromagnetic wave propagating along the z-direction is shown below

Electromagnetic waves class 12 Notes
A plane EM wave traveling along Z-axis

The electric field Ex is along the x-direction and the dotted curve shows magnetic field B which is along the y-direction. Both E and B vary sinusoidally and become maximum at same position and time. As E and B are mutually perpendicular to each other, so they are transverse in nature.

The EM wave propagating in the positive z-direction is represented as

E = Ex = E0 sin(kx – ωt), B = By = B0 sin(ky – ωt)

where k is propagation vector or wave vector = 2π/λ and ω angular frequency = 2πv

Important Characteristics of Electromagnetic waves class 12 Notes

Some features of EM waves are given below

  1. EM waves do not require any material medium for propagation.
  2. These waves travel in free space with the speed of light (3 x 108 ms-1), given by c =1 0 0 / m e , which shows that light waves are electromagnetic in nature.
  3. Speed of electromagnetic wave in a medium is v =1 / me, where e and m are the permittivity and magnetic permeability of a material medium, respectively. This means the speed of EM waves in a medium depends on electric and magnetic properties of a medium.
  4. The direction of variations of electric and magnetic fields are perpendicular to each other and also perpendicular to the direction of wave propagation. Thus, electromagnetic waves are transverse in nature.
  5. In free space, the magnitudes of electric and magnetic fields in electromagnetic waves are related by E B c 0 0 / = .
  6. The energy in electromagnetic waves is divided, on average, equally between electric and magnetic fields. U U e m = where, Ue = energy of electric field and Um = energy of the magnetic field.
  7. The energy density (energy per unit volume) in an electric field Ein vacuum is 1
  8. Electromagnetic waves, being uncharged, are not deflected by electric and magnetic fields.
  9. An electromagnetic wave carries energy and momentum. An electromagnetic wave also exerts pressure called radiation pressure. If wave is incident on a completely absorbing surface, then momentum delivered is given by

Electromagnetic Spectrum

The orderly arrangement of EM waves in increasing or decreasing order of wavelength l and frequency n is called the electromagnetic spectrum. The range varies from 10 -12 m to 104 m, i.e. from g-rays to radio waves.

The electromagnetic wave spectrum is shown below :

Electromagnetic waves class 12 Notes
Electromagnetic spectrum with common names for various parts of it

Various Electromagnetic Waves of Electromagnetic Spectrum with their Features and Uses

Name of WaveFrequency RangeWavelength RangeProductionDetectionUses
Radio waves500 kHz to
1000 MHz
> 0.1 mRapid acceleration
and decelerations of
electrons in aerials
Receiver’s aerials
  • These are used in AM (Amplitude Modulation) from 530 kHz to 1710 kHz and ground wave propagation
  • These are used in TV waves ranging from 54 MHz to 890 MHz.
  • These are used in FM (Frequency Modulation) ranging from 88 MHz to 108 MHz.
Microwaves1 GHz to
300 GHz
0.1 m to
1 mm
Klystron valve or
magnetron valve.
Point contact diodes
  • These are used in RADAR systems for aircraft navigation
  • These are used in microwave oven for cooking purpose.
  • These are used in study of atomic and molecular structures.
Infrared waves(heat waves)3 x 1011 Hz to 4 x 1014 Hz1 mm to
700 nm
The vibration of atoms and molecules.Thermopiles bolometer and infrared photographic film
  • These are used in physical therapy.
  • These are used in satellites for army purpose.
  • These are used in weather forecasting.
Light rays
(visible rays)
4 x 1014 Hz to 7 x 1014 Hz700 nm to
400 nm
Electrons in atoms emit light when they move from a higher energy level to a lower energy level.Eye, photocells, and photographic film
  • These are used by the optical organs of humans and animals for three primary purposes given below
    1. To see things, avoid bumping into them and escape danger.
    2. To look for food.
    3. To find other living things with which to copulate, so as to prolong the species.
Ultraviolet
rays
1014 Hz to
1016 Hz
400 nm
to 1 nm
Inner shell electrons in atoms move from higher energy level to a lower energy levelPhotocells and
photographic film
  • These are used in burglar alarm.
  • These are used in checking mineral sample.
  • These are used to study molecular structure.
  • To kill germs in water purifiers.
  • Used in LASER eye surgery.
X-rays3 x 1016 Hz to 3 x 1021 Hz1 nm to
10-3 nm
X-ray tubes or inner shell electrons, bombarding metals by high-energy electrons.Photographic film, Geiger tubes and ionisation chamber
  • These are used in medicine to detect the fracture, diseased organs, stones in the body, etc.
  • These are used in engineering to detect fault, cracks in bridges and testing of welds.
  • These are used at metro-stations to detect metals or explosive material.
Gamma (γ) g
rays
3 x 1018 Hz to 5 x 1022 Hz<10-3 nmRadioactive decay of the nucleus.photographic film and ionization chamber
  • These are used to produce nuclear reactions.
  • These are used in radio therapy for the treatment of tumour and cancer.
  • These are used in food industry to kill pathogenic micro-organism.
Electromagnetic Waves of Electromagnetic Spectrum with their Features and Uses

Electromagnetic radiation

In physics, Electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, propagating through space, carrying electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visible) light, ultraviolet, X-rays, and gamma rays. All of these waves form part of the electromagnetic spectrum.

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Important Question : Electromagnetic waves class 12 Notes

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