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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

Source of electromagnetic waves - Maxwell’s theory of electromagnetic radiation

A stationary charge produces only electrostatic fields, while the charges in uniform motion (steady currents) produce magnetic fields that do not vary with time.

According to Maxwell’s theory accelerated charges radiate electromagnetic waves.

Let us consider a charge oscillating with some frequency (it is an accelerating charge). This produces an oscillating electric field in space, which produces an oscillating magnetic field, which in turn, is a source of oscillating electric field, and so on. The oscillating electric and magnetic fields thus regenerate each other, as the wave propagates through the space.

The frequency of the electromagnetic wave equals the frequency of oscillation of the charge. The energy associated with the propagating wave comes at the expense of the energy of the source – the accelerated charge.

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

It is not possible to set up an ac circuit in which the current oscillate at the frequency of visible light. The frequency of yellow light is about 6×1014 Hz, while the maximum frequency of even with modern electronic circuits is about 1011 Hz. This is why the experimental demonstration of electromagnetic wave could not be proved for light. Hertz in his experiment (1887) proved this for in the low frequency region of electromagnetic waves (the radio wave region).

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