Electromagnetic Waves | Class 12 Physics Notes
By ConceptScroll Team · Published on 17 July 2026 · 5 min read
Electromagnetic Waves – this guide gives you a concise, exam-ready overview of Electromagnetic Waves from Class 12 Physics, written by ConceptScroll editors and reviewed against the latest NCERT textbook.
8.4 ELECTROMAGNETIC SPECTRUM
Maxwell's prediction of electromagnetic waves initially pertained only to visible light, but subsequent discoveries expanded the known spectrum to include ultraviolet, infrared, X-rays, gamma rays, microwaves, and radio waves. The electromagnetic spectrum encompasses all types of electromagnetic radiation, classified according to their wavelengths or frequencies. There is no sharp boundary between different regions; the classification is based on how the waves are produced and detected.
The spectrum ranges from gamma rays with wavelengths as short as 10^-12 m to long radio waves with wavelengths up to 10^6 m. Different types of electromagnetic waves have diverse applications and interactions with matter.
Radio waves are produced by accelerated charges in antennas and are used in communication systems such as radio, television, and cellular phones. Their frequencies range from about 500 kHz to 1000 MHz.
Microwaves, with frequencies in the GHz range, are generated by devices like klystrons and magnetrons. They are used in radar systems, microwave ovens, and wireless communication.
Infrared waves are emitted by hot bodies and molecules and are often called heat waves. They play a role in physical therapy, remote controls, and the greenhouse effect.
Visible light is the portion detectable by the human eye, ranging from about 400 nm to 700 nm. It provides information about the environment and is essential for vision.
Ultraviolet rays have shorter wavelengths than visible light and are produced by hot bodies and special lamps. The ozone layer absorbs most UV radiation from the sun, protecting life on Earth. UV rays are used in sterilization and medical applications.
X-rays, with even shorter wavelengths, are generated by bombarding metal targets with high-energy electrons. They are widely used in medical imaging and cancer treatment.
Gamma rays have the shortest wavelengths and highest frequencies, produced in nuclear reactions and radioactive decay. They are used in medical treatments and scientific research.
Table 8.1 summarizes the types of electromagnetic waves, their wavelength ranges, production methods, and detection techniques.
📊 Diagram: See figure_9: The electromagnetic spectrum, with common names for various parts of it. The various regions do not have sharply defined boundaries.
🔗 Connection: This section provides context for understanding the diverse applications of electromagnetic waves, leading to exercises that reinforce the concepts.
Table on page 11 (4×4)
| Type | Wavelength range | Production | Detection |
|---|---|---|---|
| Radio | >0.1 m | Rapid acceleration and decelerations of electrons in aerials | Receiver's aerials |
| Microwave | 0.1m to 1 mm | Klystron valve or magnetron valve | Point contact diodes |
| Infra-red | 1mm to 700 nm | Vibration of atoms and molecules | Thermopiles
Frequently asked questions
8.1 Figure 8.5 shows a capacitor made of two circular plates each of radius 12 cm, and separated by 5.0 cm. The capacitor is being charged by an external source (not shown in the figure). The charging current is constant and equal to 0.15 A. (a) Calculate the capacitance and the rate of change of potential difference between the plates. (b) Obtain the displacement current across the plates. (c) Is Kirchhoff's first rule (junction rule) valid at each plate of the capacitor? Explain.
Given: Radius of plates, r = 12 cm = 0.12 m Separation between plates, d = 5.0 cm = 0.05 m Charging current, I = 0.15 A
(a) Calculate capacitance C: Capacitance of parallel plate capacitor, C = ε₀ A / d Area, A = π r² = π × (0.12)² = π × 0.0144 = 0.04524 m² ε₀ = 8.854 × 10⁻¹² F/m So, C = (8.854 × 10⁻¹²) × 0.04524 / 0.05 = 8.01 × 10⁻¹² F = 8.01 pF
Rate of change of potential difference (dV/dt): Current I = C (dV/dt) ⇒ dV/dt = I / C = 0.15 / (8.01 × 10⁻¹²) = 1.87 × 10¹⁰ V/s
(b) Displacement cur
8.2 A parallel plate capacitor (Fig. 8.6) made of circular plates each of radius R = 6.0 cm has a capacitance C = 100 pF. The capacitor is connected to a 230 V ac supply with a (angular) frequency of 300 rad s⁻¹. (a) What is the rms value of the conduction current? (b) Is the conduction current equal to the displacement current? (c) Determine the amplitude of B at a point 3.0 cm from the axis between the plates.
Given: Radius R = 6.0 cm = 0.06 m Capacitance C = 100 pF = 100 × 10⁻¹² F Voltage amplitude V₀ = 230 V (rms given, so V_rms = 230 V) Angular frequency ω = 300 rad/s
(a) RMS conduction current I_rms: For capacitor, current leads voltage by 90° I = C dV/dt = C ω V₀ cos(ω t) RMS current I_rms = ω C V_rms I_rms = 300 × 100 × 10⁻¹² × 230 = 6.9 × 10⁻⁶ A = 6.9 μA
(b) The conduction current in the wires is equal in magnitude to the displacement current between the plates because the displacement curren
8.3 What physical quantity is the same for X-rays of wavelength 10⁻¹⁰ m, red light of wavelength 6800 Å and radiowaves of wavelength 500 m?
The physical quantity that is the same for all electromagnetic waves, regardless of their wavelength, is the speed of the wave in vacuum. All electromagnetic waves travel at the speed of light, c = 3 × 10⁸ m/s.
8.4 A plane electromagnetic wave travels in vacuum along z-direction. What can you say about the directions of its electric and magnetic field vectors? If the frequency of the wave is 30 MHz, what is its wavelength?
In a plane electromagnetic wave traveling along the z-direction, the electric field vector (E) and magnetic field vector (B) are perpendicular to each other and both are perpendicular to the direction of propagation (z-axis). Thus, E and B lie in the x-y plane.
Frequency, ν = 30 MHz = 30 × 10⁶ Hz Wavelength, λ = c / ν = (3 × 10⁸ m/s) / (30 × 10⁶ Hz) = 10 m
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