Revision: Class 12 >> Displacement current and Electromagnetic Waves NEET (UG) Displacement current and Electromagnetic Waves

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).

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 the electromagnetic waves arranged in ascending or descending order is known as the electromagnetic 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.

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

\[\frac {dE}{dt}\] ≠ 0 ⇒ i_d​ exists

\[v_p=\frac{\omega}{k}=\lambda f\]

\[v_g=\frac{d\omega}{dk}\]

\[v_s=\frac{1}{\sqrt{\mu\varepsilon}}\]

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

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

• The visible spectrum lies between violet and red and is only a small part of the electromagnetic spectrum, while regions beyond are called the invisible spectrum.
• The electromagnetic spectrum, in increasing order of wavelength, is: gamma rays, X‑rays, ultraviolet rays, visible light, infrared radiation, microwaves, and radio waves.
• All electromagnetic waves travel with the same speed (3 × 10⁸ m s⁻¹) in vacuum, and they are transverse waves that show reflection and refraction without a change in frequency.