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

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

The current associated with the time rate of change of electric flux is called displacement current.

\[i_d=\varepsilon_0\frac{d\Phi_E}{dt}\]

The self-sustaining oscillations of electric and magnetic fields which propagate through space are called electromagnetic waves.

Electromagnetic waves produced by bombarding a metal target with high energy electrons are called X-rays.

Short wavelength radio waves with frequencies in gigahertz range used in radar and communication systems are called microwaves.

Electromagnetic waves produced by hot bodies and molecules lying beyond the red end of visible light are called infrared waves.

The portion of electromagnetic spectrum detected by the human eye between about 400 nm to 700 nm is called visible light.

Electromagnetic waves with wavelengths shorter than visible light but longer than X-rays are called ultraviolet rays.

The high frequency electromagnetic waves produced in nuclear reactions and radioactive decay are called gamma rays.

The range of all electromagnetic waves arranged according to their frequency or wavelength is called electromagnetic spectrum.

Electromagnetic waves having wavelengths greater than 0.1 m produced by accelerated charges in conducting wires are called radio waves.

• Gauss’s Law states that the total electric flux through any closed surface is proportional to the total charge enclosed within the surface.
• The electric flux depends only on the charge enclosed, not on the shape or size of the surface.
• Charges outside the closed surface do not contribute to the net electric flux.
• It establishes a relation between electric field and electric charge.
• It is one of Maxwell’s fundamental equations.
• It is useful in calculating electric field for symmetrical charge distributions.
• Mathematically, it is expressed as: \[\oint\mathbf{E}\cdot d\mathbf{A}=\frac{Q}{\varepsilon_0}\]

• Gauss’s Law for magnetism states that the total magnetic flux through any closed surface is zero.
• It implies that magnetic monopoles do not exist in nature.
• Magnetic field lines always form closed loops.
• The number of magnetic field lines entering a surface equals the number leaving it.
• Magnetic field has no isolated source like electric charge.
• It is one of Maxwell’s equations.
• Mathematically, it is expressed as: ∮B⋅dA = 0

• Faraday’s Law states that a changing magnetic flux produces an induced emf in a closed circuit.
• The magnitude of induced emf is equal to the rate of change of magnetic flux.
• The induced electric field forms closed loops.
• The negative sign indicates Lenz’s Law (direction of induced emf).
• It shows that a time-varying magnetic field produces an electric field.
• It forms one of Maxwell’s equations.
• Mathematically, it is expressed as: \[\oint\mathbf{E}\cdot d\mathbf{l}=-\frac{d\Phi_B}{dt}\]

• Ampere–Maxwell Law states that magnetic field is produced by both conduction current and displacement current.
• Maxwell introduced displacement current to remove inconsistency in Ampere’s circuital law.
• Displacement current arises due to time-varying electric field.
• The total current is the sum of conduction current and displacement current.
• It establishes symmetry between electric and magnetic fields.
• It explains magnetic field inside a charging capacitor.
• Mathematically, it is expressed as: \[\oint\mathbf{B}\cdot d\mathbf{l}=\mu_0i_c+\mu_0\varepsilon_0\frac{d\Phi_E}{dt}\]

• 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.

• Electromagnetic waves are produced by accelerated charges; neither stationary charges nor charges in uniform motion can produce electromagnetic waves.
• An oscillating charge produces an oscillating electric field, which in turn produces an oscillating magnetic field, and the two fields continuously regenerate each other.
• The electric field and magnetic field in an electromagnetic wave are perpendicular to each other and also perpendicular to the direction of propagation.
• Electromagnetic waves are self-sustaining oscillations of electric and magnetic fields and do not require any material medium for propagation.
• The frequency of an electromagnetic wave is equal to the frequency of oscillation of the accelerating charge that produces it.
• The speed of electromagnetic waves in free space is \[c=\frac{1}{\sqrt{\mu_0\varepsilon_0}}=3\times10^8\mathrm{m/s}\] which is independent of wavelength.
• The magnitudes of electric and magnetic fields in an electromagnetic wave are related by \[B_0=\frac{E_0}{c}\].
• In a material medium of permittivity ε and permeability μ, the velocity of electromagnetic waves is \[v=\frac{1}{\sqrt{\mu\varepsilon}}\].