# Notes And Questions NCERT Class 12 Physics Chapter 8 Electromagnetic Waves

Please refer to Electromagnetic Waves Class 12 Physics Notes and important questions below. The Class 12 Physics Chapter wise notes have been prepared based on the latest syllabus issued for the current academic year by CBSE. Students should revise these notes and go through important Class 12 Physics examination questions given below to obtain better marks in exams

## Electromagnetic Waves Class 12 Physics Notes and Questions

The below Class 12 Electromagnetic Waves notes have been designed by expert Physics teachers. These will help you a lot to understand all the important topics given in your NCERT Class 12 Physics textbook. Refer to Chapter 8 Electromagnetic Waves Notes below which have been designed as per the latest syllabus issued by CBSE and will be very useful for upcoming examinations to help clear your concepts and get better marks in examinations.

• Displacement Current:
It is due to time-varying electric field is,

Displacement current acts as a source of magnetic field in exactly the same way as conduction current.

• Electromagnetic Waves:
a) Electromagnetic waves are produced only by charges that are accelerating, since acceleration is absolute, and not a relative phenomenon.
b) An electric charge oscillating harmonically with frequency v, produces electromagnetic waves of the same frequency v.
c) An electric dipole is a basic source of electromagnetic waves.
d) Electromagnetic waves with wavelength of the order of a few metres were first produced and detected in the laboratory by Hertz in 1887. He thus verified a basic prediction of Maxwell’s equations.

• Oscillation of Electric and Magnetic Fields:
These 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 v ,wavelength λ  propagating along z-direction,

• Relation between μo and ∈0 :
The speed c of electromagnetic wave in vacuum is related to 11o and E0 (the free space permeability and permittivity constants) as

•    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.

• Speed of Light:
The speed of light, or of electromagnetic waves in a material medium is

Where p is the permeability of the medium and E its permittivity

• Electromagnetic waves carry energy as they travel through space and this energy is shared equally by the electric and magnetic fields.

• Energy Per Unit Volume:

where we are assuming that the concerned space consists of vacuum only.

• Electromagnetic waves transport momentum as well. When these waves strike a surface, a pressure is exerted on the surface.
• If total energy transferred to a surface in time tis U, total momentum delivered to this surface is p = UIc.

• Electromagnetic Spectrum:
The spectrum of electromagnetic waves stretches, in principle, over an infinite range of wavelengths. The classification of electromagnetic waves according to frequency is the electromagnetic spectrum. There is no sharp division between one kind of wave and the next.
• The classification has more to do with the way these waves are produced and detected.

• Different Regions of Spectrum:
Different regions are known by different names; y -rays, X-rays, ultraviolet rays, visible rays, infrared rays, microwaves and radio waves in order of increasing wavelength from
10-2 A or 10-12 m to 106m.

✔ These are produced by accelerated motion of charges in wires.
✔ These are used in radio and television communication systems.
✔ These are generally in the frequency range from 500 kHz to about 1000 MHz. (b)Microwaves:
✔ These are short wavelength radio waves with frequencies in the gigahertz range.
✔ Due to their short wavelengths, they are suitable for radar systems used in aircraft navigation.
✔ Microwave ovens use them for cooking. (c) Infrared Waves:
✔ These are produced by hot bodies and molecules.
✔ They lie in the low frequency or long wavelength end of the visible spectrum.

(d) Visible Light: