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.
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
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
- EM waves do not require any material medium for propagation.
- 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.
- 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.
- 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.
- In free space, the magnitudes of electric and magnetic fields in electromagnetic waves are related by E B c 0 0 / = .
- 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.
- The energy density (energy per unit volume) in an electric field Ein vacuum is 1
- Electromagnetic waves, being uncharged, are not deflected by electric and magnetic fields.
- 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
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 :
Various Electromagnetic Waves of Electromagnetic Spectrum with their Features and Uses
|Name of Wave||Frequency Range||Wavelength Range||Production||Detection||Uses|
|Radio waves||500 kHz to|
|> 0.1 m||Rapid acceleration|
and decelerations of
electrons in aerials
|Microwaves||1 GHz to|
|0.1 m to|
|Klystron valve or|
|Point contact diodes|
|Infrared waves(heat waves)||3 x 1011 Hz to 4 x 1014 Hz||1 mm to|
|The vibration of atoms and molecules.||Thermopiles bolometer and infrared photographic film|
|4 x 1014 Hz to 7 x 1014 Hz||700 nm to|
|Electrons in atoms emit light when they move from a higher energy level to a lower energy level.||Eye, photocells, and photographic film|
|1014 Hz to|
to 1 nm
|Inner shell electrons in atoms move from higher energy level to a lower energy level||Photocells and|
|X-rays||3 x 1016 Hz to 3 x 1021 Hz||1 nm to|
|X-ray tubes or inner shell electrons, bombarding metals by high-energy electrons.||Photographic film, Geiger tubes and ionisation chamber|
|Gamma (γ) g|
|3 x 1018 Hz to 5 x 1022 Hz||<10-3 nm||Radioactive decay of the nucleus.||photographic film and ionization chamber|
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.