Revision: Electromagnetic Waves >> Electromagnetic Waves Physics Science (English Medium) Class 12 CBSE

Electromagnetic waves are self-propagating transverse waves consisting of oscillating electric and magnetic fields that do not require a medium for propagation.

The whole range of frequencies/wavelengths of electromagnetic waves arranged in ascending or descending order is known as the electromagnetic spectrum.

The current that exists at any point in space where a time-varying electric field (E) exists, i.e., \[\frac {dE}{dt}\] ≠ 0, is called displacement current (iₐ).

The whole range of frequencies/wavelengths of the electromagnetic waves arranged in ascending or descending order is known as the electromagnetic spectrum.

The part of the spectrum beyond the red extreme and the violet extreme is called the invisible spectrum.

Electromagnetic waves have a wide range of frequencies and wavelengths. We can see only a small portion of the electromagnetic spectrum, which is known as the visible spectrum.

\[\frac {dE}{dt}\] ≠ 0 ⇒ i_d​ exists

\[\oint_c\vec{B}\cdot d\vec{l}=\mu_0I_c+\varepsilon_0\mu_0\frac{d\Phi_E}{dt}\]

This equation states that not only the current but also the changing electric field produces a changing magnetic field. This equation is known as the Ampere-Maxwell Circuital Law.

• EM waves are transverse
• E ⟂ B ⟂ direction of propagation
• E and B are in phase
• Speed:
  \[c=\frac{1}{\sqrt{\mu_0\varepsilon_0}}=3\times10^8\mathrm{m/s}\]
• Poynting Vector:
  \[\vec{S}=\frac{\vec{E}\times\vec{B}}{\mu_{0}}\]

• Intensity:
  \[I=\frac{1}{2}c\varepsilon_0E_0^2\]

• Electric and magnetic fields in an electromagnetic wave are perpendicular to each other.
• Both fields are perpendicular to the direction of propagation.
• Electromagnetic waves are self-sustaining oscillations in a vacuum.
• No material medium is required for their propagation.
• In a vacuum, their speed is 3 × 10⁸ m/s.
• In a material medium, velocity is \[\frac {1}{\sqrt {με}}\].
• Radio, TV signals, and light are important examples of energy transfer by electromagnetic waves.

• The electromagnetic spectrum is the complete range of electromagnetic radiation.
• Order of EM waves: Radio → Microwave → Infrared → Visible → Ultraviolet → X-ray → Gamma.
• All EM waves travel at the speed 3 × 108 m/s in a vacuum.
• Frequency increases from radio waves to gamma rays.
• Wavelength decreases from radio waves to gamma rays.
• Visible light is only a small part of the full spectrum.