## ELECTROMAGNETIC WAVES

 Introduction to electromagnetic waves Displacement current Ampere – Maxwell law Ampere’s circuital law Displacement current Electromagnetic waves Sources of electromagnetic waves Nature of electromagnetic waves Electromagnetic spectrum Radio Waves Microwaves Infrared waves Visible rays X rays Gamma rays

## Displacement current:

Maxwell found an inconsistency in the Ampere’s law and suggested the existence of an additional current, called displacement current, to remove this inconsistency. This displacement current is due to time-varying electric field and is given by and acts as a source of magnetic field in exactly the same way as conduction current.
An ac voltage v = vm sinωt applied to a capacitor drives a current in the capacitor:
i = im sin (ωt + π/2)
Here, it is called capacitive reactance. The average power supplied to a capacitor over one complete cycle is zero.
An accelerating charge produces electromagnetic waves. An electric charge oscillating harmonically with frequency , produces electromagnetic waves of the same frequency . An electric dipole is a basic source of electromagnetic waves.
Electric and magnetic fields oscillate sinusoidally in space and time in an electromagnetic wave. The oscillating electric and magnetic fields, E and B are perpendicular to each other, and to the direction of propagation of the electromagnetic wave. For a wave of frequency , wavelength λ, propagating along z-direction, we have The speed c of electromagnetic wave in vacuum is related to μ0 and ε0 (the free space permeability and permittivity constants) as follows:

• c = 1/√ (μ0ε0). The value of c equals the speed of light obtained from optical measurements.
• Light is an electromagnetic wave; c is, therefore, also the speed of light.
• Electromagnetic waves other than light also have the same velocity c in free space.
• The speed of light, or of electromagnetic waves in a material medium is given by,
v = 1/√(με)
where μ is the permeability of the medium and ε its permittivity.
The spectrum of electromagnetic waves stretches, in principle, over an infinite range of wavelengths. Different regions are known by different names; γ-rays, X-rays, ultraviolet rays, visible rays, infrared rays, microwaves and radio waves in order of increasing wavelength from 10–2 Å or 10–12 m to 106

• They interact with matter via their electric and magnetic fields which set in oscillation charges present in all matter. The detailed interaction and so the mechanism of absorption, scattering, etc., depend on the wavelength of the electromagnetic wave, and the nature of the atoms and molecules in the medium.