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CBSE NOTES CLASS 12 PHYSICS

CHAPTER 8 ELECTROMAGNETIC WAVES

Displacement current - the missing term in Ampere’s circuital law

Maxwell’s generalisation of Ampere’s circuital law

Maxwell equations

Source of electromagnetic waves

Maxwell’s theory of electromagnetic radiation

Why can’t we prove that light is an electromagnetic wave?

Nature of electromagnetic waves

Properties of electromagnetic waves

Speed of electromagnetic wave

Poynting vector

Energy in electromagnetic waves

Radiation pressure

Electromagnetic spectrum

Radio waves

Microwaves

CBSE NOTES CLASS 12 PHYSICS

CHAPTER 8 ELECTROMAGNETIC WAVES

Nature of electromagnetic waves

Electromagnetic waves are combination of oscillating magnetic and electrical fields which are mutually perpendicular to each other and also perpendicular to the propagation of the wave. Electric and magnetic field vectors change sinusoidally.

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If the propagation of wave is taken to be in z direction, Electrical field along x direction and Magnetic field along y directions, then the equation of plane progressive electromagnetic wave can be written as

Ex= Eo sin (kzωt )

and By= Bo sin (kzωt), 

Here ω =2πν is the angular frequency and k = 2πλ is the magnitude of the wave vector (or propagation vector) k. Direction of k describes the direction of propagation of the wave.

We can see that speed of propagation of the wave is

c= λν=ω2π×2πk=ωk

ω = c k,  where c =1μoεo 

The magnitude of the electric and the magnetic fields in an electromagnetic wave are related as

c= EoBo

Where Eo and Bo are maximum values of electric and magnetic field vectors.

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