CBSE Class 12 Physics Notes Chapter 8 Electromagnetic Waves

What Is Displacement Current?

An inconsistency in the ampere law and the presence of an additional current in order to remove this inconsistency is referred to as displacement current. The displacement current acts as a source of the magnetic field, just like the conduction current.

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Spectrum of Electromagnetic Waves

The spectrum of electromagnetic waves ranges over different wavelengths. Those are:

  • Infrared Rays
  • Visible Rays
  • Ultraviolet Rays
  • X- Rays
  • Radio Waves
  • Microwaves

They interact with matter through their electric and magnetic fields, which are responsible for the oscillations charges of the matter. The mechanism of scattering and adsorption depends on electromagnetic waves’ wavelength along with the nature of the atoms and molecules.

Total Energy of an Electromagnetic Wave

Electromagnetic waves possess energy when they move through space, and they share this energy through their magnetic and electric fields. These waves are also responsible for transporting momentum also. With the strike of total energy on a surface, pressure is seen to be exerted on the surface. If the total energy transferred to a surface is taken as U and in the time t, then the total momentum delivered to the surface is p = U/c.

The speed of an electromagnetic wave in the vacuum medium is

\(\begin{array}{l}c=1/\sqrt{\mu _{0}\varepsilon _{0}}\end{array} \)

Here, the value of c equals the speed of light from the measuring optical instruments. Light is also an electromagnetic wave, which means it has the same speed as light. Even electromagnetic waves other than light also have the same speed as light in the free medium and is given by

\(\begin{array}{l}\upsilon = 1/\sqrt{\mu \varepsilon }\end{array} \)
\(\begin{array}{l}\mu\end{array} \)
is the permeability of the considered medium and
\(\begin{array}{l}\varepsilon\end{array} \)
is the permittivity.

A Dipole is the basic component of an electromagnetic wave. An accelerating charge produces an electromagnetic wave.

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Important Questions

  1. By using the formula E = hν, obtain the photon energy in units of eV for different parts of the electromagnetic spectrum (for the energy of a quantum of radiation: photon).
  2. What can you say about the directions of the electric and magnetic field vectors of a plane electromagnetic wave t which is travelling in vacuum along z-direction and the frequency of the wave is 50 MHz and find out what its wavelength is?
  3. A radio can tune in to any station in the 8.3 MHz to 15 MHz bands. What is the corresponding wavelength band?

Frequently Asked Questions on CBSE Class 12 Physics Notes Chapter 8 Electromagnetic Waves


What are the uses of infrared rays?

Infrared rays are widely used for cosmetic applications such as treating skin injuries, smoothing wrinkles, reducing the occurrence of dandruff, blackheads, etc.


What are the uses of electromagnetic waves?

Electromagnetic waves have a vast range of practical everyday applications that includes such diverse uses as communication by cell phone and radio broadcasting, WiFi, cooking, vision, medical imaging, and treating cancer.


What is a displacement current?

The displacement current was named as current because it is similar to the conduction current. Displacement current is the current due to the changing of the electric field inside the plate of the capacitor.


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