# Electromagnetic waves class 12 Notes, Ncert solutions, important questions

0
105

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.

## 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

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 :