RBSE Class 12 Physics Chapter 8, titled Electromagnetic Waves (विद्युतचुम्बकीय तरंगें), is a crucial chapter that introduces you to the fundamental nature of light and all other forms of electromagnetic (EM) radiation. Although it carries a small weightage of 2 marks in the total 56-mark theory paper, the concepts are foundational for later chapters, particularly Optics.
Here is a complete breakdown of the syllabus, key concepts, and important formulas you need to master this chapter.
📚 Syllabus Breakdown: Key Topics
According to the official Rajasthan Board (RBSE) Class 12 Physics Syllabus, Chapter 8 primarily focuses on three main areas:
- Displacement Current (विस्थापन धारा):
- Need for Displacement Current.
- Maxwell’s modification of Ampere’s Circuital Law (Ampere-Maxwell Law).
- Electromagnetic Waves (विद्युतचुम्बकीय तरंगें):
- Production and properties of EM waves.
- Nature of EM waves (Transverse nature).
- Speed of EM waves in vacuum and medium.
- Electromagnetic Spectrum (विद्युतचुम्बकीय स्पेक्ट्रम):
- Classification and properties of different parts of the spectrum.
- Basic applications of Radio waves, Microwaves, Infrared, Visible light, Ultraviolet, X-rays, and Gamma rays.
🔑 Core Concepts to Master
1. Displacement Current ($I_D$)
The concept of displacement current was introduced by Maxwell to ensure the consistency of Ampere’s circuital law when dealing with time-varying electric fields, such as those inside a charging capacitor.
- Maxwell-Ampere Law (Complete form of Ampere’s Law):$$\oint \vec{B} \cdot d\vec{l} = \mu_0 \left( I_C + I_D \right)$$Where $I_C$ is the conduction current and $I_D$ is the displacement current.
- Displacement Current Formula:$$I_D = \epsilon_0 \frac{d\Phi_E}{dt}$$Where $\frac{d\Phi_E}{dt}$ is the rate of change of electric flux. This shows that a changing electric field generates a magnetic field.
2. Production and Properties of EM Waves
Electromagnetic waves are produced by accelerated or oscillating charges.
- Production: An oscillating electric charge generates a time-varying electric field, which in turn generates a time-varying magnetic field (Displacement Current), and this cycle continues, sustaining the propagation of the wave.
- Transverse Nature: The electric field ($\vec{E}$) and magnetic field ($\vec{B}$) vectors oscillate perpendicular to each other and perpendicular to the direction of wave propagation.
- Speed in Vacuum ($c$): EM waves travel at the speed of light in vacuum, which is determined by the electrical and magnetic properties of free space:$$c = \frac{1}{\sqrt{\mu_0 \epsilon_0}}$$Where $\mu_0$ is the permeability of free space and $\epsilon_0$ is the permittivity of free space.
- Field Relation: The magnitudes of the electric and magnetic field amplitudes ($E_0$ and $B_0$) are related by the speed of light:$$c = \frac{E_0}{B_0}$$
- No Medium Required: They do not require any material medium for propagation.
3. Electromagnetic Spectrum
The EM spectrum is the orderly distribution of electromagnetic radiation according to its wavelength ($\lambda$) or frequency ($\nu$).
| Region | Wavelength Range (Approx.) | Production/Source | Primary Application |
| Gamma Rays ($\gamma$) | $< 10^{-12}$ m | Nuclear decay | Cancer treatment (radiotherapy) |
| X-rays | $10^{-11} \text{ to } 10^{-8}$ m | Electron bombardment of metal targets | Medical imaging (fracture detection) |
| UV Rays | $10^{-8} \text{ to } 4 \times 10^{-7}$ m | Very hot bodies, special lamps | Sterilizing medical instruments, water purifiers |
| Visible Light | $4 \times 10^{-7} \text{ to } 7 \times 10^{-7}$ m | Atomic excitation/de-excitation | Vision, Photography |
| Infrared (IR) | $7 \times 10^{-7} \text{ to } 10^{-3}$ m | Hot bodies (vibrating atoms/molecules) | Remote controls, Thermal imaging (night vision) |
| Microwaves | $10^{-3} \text{ to } 0.1$ m | Klystron/Magnetron tubes | Cooking (Microwave oven), Radar systems |
| Radio Waves | $> 0.1$ m | Accelerated movement of charge in wires (Antennas) | Communication (AM/FM radio, TV broadcast) |
📝 Important Formulas and Relations
| Concept | Formula | Relation |
| Displacement Current | $I_D = \epsilon_0 \frac{d\Phi_E}{dt}$ | Time-varying Electric Flux $\rightarrow$ Magnetic Field |
| Speed of EM Wave ($c$) | $c = \frac{1}{\sqrt{\mu_0 \epsilon_0}}$ | $\mu_0$ (Permeability of Free Space), $\epsilon_0$ (Permittivity of Free Space) |
| Speed in Medium ($v$) | $v = \frac{1}{\sqrt{\mu \epsilon}}$ | $\mu$ (Permeability of Medium), $\epsilon$ (Permittivity of Medium) |
| Field Amplitudes | $c = \frac{E_0}{B_0}$ | $E_0$ (Electric Field Amplitude), $B_0$ (Magnetic Field Amplitude) |
| Wavelength-Frequency | $c = \nu \lambda$ | $\nu$ (Frequency), $\lambda$ (Wavelength) |
| Average Energy Density | $u_{avg} = u_E + u_B = \frac{1}{2}\epsilon_0 E^2 + \frac{1}{2\mu_0} B^2$ | Energy is shared equally between $\vec{E}$ and $\vec{B}$ fields ($u_E = u_B$). |