Appeared in: 2022 (3 sets)
(a) (i) Depict a plane electromagnetic wave propagating along the x-axis. Write the expressions for its oscillating electric and magnetic fields. (ii) Write three characteristics of electromagnetic waves. OR (b) Name the electromagnetic waves which are produced by the following : (i) Radioactive decays of nucleus (ii) Welding arcs (iii) Hot bodies Write one use each of these waves.
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Official Evaluation Steps & EL10 Insight
Solution for CBSE Class 12 Physics: Electromagnetic Waves
Part (a)
(i) Representation and Expressions:
- Step 1 (Conceptual Representation): [0.5 Mark] The electric field vector $\vec{E}$ oscillates along the y-axis and the magnetic field vector $\vec{B}$ oscillates along the z-axis for a wave propagating along the x-axis.
- Step 2 (Mathematical Expressions): [0.5 Mark] The expressions for the oscillating fields are: $$E_y = E_0 \sin(kx - \omega t)$$ $$B_z = B_0 \sin(kx - \omega t)$$ Where $E_0$ and $B_0$ are the amplitudes of the electric and magnetic fields respectively.
(ii) Characteristics of EM Waves: [0.5 Mark]
- EM waves are transverse in nature.
- They do not require any material medium for propagation.
- They travel in free space with the speed of light, $c = \frac{1}{\sqrt{\mu_0 \epsilon_0}} \approx 3 \times 10^8 \text{ m/s}$.
Part (b)
Identification and Uses: [0.5 Mark]
| Source | Electromagnetic Wave | Use |
|---|---|---|
| (i) Radioactive decay | Gamma rays ($\gamma$-rays) | Used in radiotherapy to treat cancer cells. |
| (ii) Welding arcs | Ultraviolet (UV) rays | Used in water purifiers to kill germs/bacteria. |
| (iii) Hot bodies | Infrared (IR) rays | Used in remote switches or night vision goggles. |
💡 EL10 Concept Insight: Think of an electromagnetic wave like a synchronized dance between electricity and magnetism: as the electric field wiggles up and down, it creates a magnetic field that wiggles side to side, and this "tag-team" allows the energy to zip through empty space at the fastest speed possible in the universe!