Class 9 Physics Chapter 8: Electromagnetic Waves Important Questions with Solutions

The CBSE Class 9 Physics syllabus emphasizes electromagnetic waves as a unifying concept in physics. Since the 2024–25 rationalization, Chapter 8 has maintained focus on Maxwell's qualitative description (the four equations in plain language) and the electromagnetic spectrum with practical applications. Board exams test: (1) conceptual understanding of E-fields and B-fields oscillating perpendicular to wave propagation, (2) ability to order spectrum by frequency/wavelength (radio > microwave > infrared > visible > UV > X-ray > gamma), (3) applications of each spectrum band in real life (mobile phones use microwaves, hospitals use X-rays), and (4) the speed relationship c = f × λ. Scoring well requires not memorizing facts but reasoning through how Maxwell unified electromagnetism. Most students lose marks by confusing frequency and wavelength order, or by not explaining why EM waves need no medium. This page drills exactly these patterns—expect 2–3 questions from EM content in your board paper, worth 6–8 marks total.

**Question 1:** An electromagnetic wave travels in vacuum at a speed of: (a) 3 × 10⁸ m/s (b) 3 × 10⁶ m/s (c) 3 × 10¹⁰ m/s (d) Depends on frequency **Answer: (a)** The speed of all electromagnetic waves in vacuum is c = 3 × 10⁸ m/s, independent of frequency or wavelength. **Question 2:** In an electromagnetic wave, the electric field and magnetic field are: (a) Parallel to each other (b) Perpendicular to each other (c) At 45° to each other (d) In the same plane **Answer: (b)** Maxwell's equations show that E and B oscillate perpendicular to each other and both perpendicular to the direction of wave propagation. **Question 3:** Which of the following has the longest wavelength? (a) Gamma rays (b) Ultraviolet light (c) Radio waves (d) X-rays **Answer: (c)** Radio waves have frequencies ~10⁶–10⁹ Hz, giving wavelengths from metres to kilometres—far longer than gamma rays (~10⁻¹² m). **Question 4:** Electromagnetic waves can travel through: (a) Vacuum only (b) Matter only (c) Both vacuum and matter (d) Neither **Answer: (c)** EM waves require no medium (they travel at 3 × 10⁸ m/s in vacuum) but can also propagate through materials at reduced speed. **Question 5:** The relationship between frequency (f) and wavelength (λ) for an EM wave is: (a) f × λ = constant (b) f / λ = constant (c) f + λ = constant (d) f – λ = 0 **Answer: (a)** The wave equation c = f × λ holds for all EM waves; f and λ are inversely proportional.

**Question 1: State any two properties of electromagnetic waves.** Answer: (1) Electromagnetic waves are transverse waves where the electric and magnetic fields oscillate perpendicular to the direction of propagation. (2) They can travel through vacuum without requiring a medium, at a constant speed of 3 × 10⁸ m/s. **Question 2: Why does an electromagnetic wave not require a medium to propagate?** Answer: According to Maxwell's equations, a changing electric field produces a magnetic field and vice versa. This mutual induction of E and B fields sustains wave propagation without needing particles in a medium. The wave is self-propagating in space. **Question 3: Arrange the following in order of increasing frequency: infrared, X-rays, visible light, microwaves.** Answer: Microwaves < Infrared < Visible light < X-rays. Frequency increases as: ~10⁹ Hz < ~10¹³ Hz < ~10¹⁵ Hz < ~10¹⁸ Hz. **Question 4: A radio wave has a frequency of 100 MHz. Calculate its wavelength (c = 3 × 10⁸ m/s).** Answer: Using c = f × λ, we get λ = c / f = (3 × 10⁸) / (100 × 10⁶) = (3 × 10⁸) / (10⁸) = 3 m. The wavelength is 3 metres. **Question 5: Name one application each of microwaves and ultraviolet radiation.** Answer: Microwaves: Mobile phone communication and microwave ovens for cooking. Ultraviolet radiation: Sterilization of medical instruments and detection of forged currency notes.

**Question 1: Explain why the sun's ultraviolet radiation is more dangerous than visible light to human skin, even though both are EM waves.** Answer: UV radiation has a higher frequency (~10¹⁶ Hz) and shorter wavelength (~100 nm) compared to visible light (~10¹⁵ Hz, ~500 nm). Higher frequency means each photon carries more energy (E = hf, where h is Planck's constant). This higher energy allows UV photons to ionize DNA molecules and damage skin cells, causing sunburn and increasing skin cancer risk. Visible light photons lack sufficient energy to ionize DNA, so they are relatively safe. **Question 2: How did Maxwell's equations unify electricity and magnetism, and what did this predict?** Answer: Maxwell showed that changing electric fields create magnetic fields and changing magnetic fields create electric fields. This mutual induction means that a pair of oscillating E and B fields can sustain each other and propagate through space as a wave. Maxwell's equations predicted that: (1) EM waves travel at speed c = 1/√(ε₀μ₀) ≈ 3 × 10⁸ m/s, (2) light is an EM wave, and (3) other EM waves exist beyond visible light (radio, X-rays, etc.). This unified the previously separate phenomena of light, electricity, and magnetism into one framework. **Question 3: Why do X-rays pass through soft tissue but are absorbed by bones, while visible light cannot pass through either?** Answer: X-rays have high frequency (~10¹⁸ Hz) and very short wavelength (~0.1 nm), allowing them to penetrate matter. Soft tissue (water, fat, muscle) is mostly transparent to X-rays because atoms in these materials have low atomic number. Bones contain calcium and phosphorus with higher atomic numbers, so they absorb X-rays strongly through interaction with core electrons. Visible light has much lower frequency (~10¹⁵ Hz) and longer wavelength (~500 nm), so it interacts strongly with the electron clouds of all biological molecules and is absorbed in just a few millimetres of tissue.

**Question 1: Describe the electromagnetic spectrum, listing all major regions in order of increasing frequency. Explain why frequency and wavelength are inversely related.** Solution: The electromagnetic spectrum, ordered by increasing frequency: 1. Radio waves: f ~ 10⁶–10⁹ Hz, λ ~ 1 cm–1 km. Uses: AM/FM radio, television. 2. Microwaves: f ~ 10⁹–10¹² Hz, λ ~ 1 mm–1 cm. Uses: Mobile phones, radar, microwave ovens. 3. Infrared (IR): f ~ 10¹²–10¹⁵ Hz, λ ~ 700 nm–1 mm. Uses: Thermal imaging, remote controls. 4. Visible light: f ~ 10¹⁵ Hz, λ ~ 400–700 nm. Red (~4.3 × 10¹⁴ Hz) to Violet (~7.5 × 10¹⁴ Hz). 5. Ultraviolet (UV): f ~ 10¹⁵–10¹⁷ Hz, λ ~ 10–400 nm. Uses: Sterilization, fluorescence. 6. X-rays: f ~ 10¹⁷–10¹⁹ Hz, λ ~ 0.01–10 nm. Uses: Medical imaging, crystallography. 7. Gamma rays: f > 10¹⁹ Hz, λ < 0.01 nm. Uses: Cancer therapy, industrial sterilization. Inverse relationship between f and λ: All EM waves travel at speed c in vacuum. From the wave equation: c = f × λ Rearranging: f = c / λ and λ = c / f This shows frequency and wavelength are inversely proportional. As frequency increases, wavelength decreases proportionally, and vice versa. For example, a radio wave at f = 10⁹ Hz has λ = 0.3 m, while a gamma ray at f = 10²⁰ Hz has λ = 3 × 10⁻¹² m—a factor of 10¹¹ difference. **Question 2: State Maxwell's four equations in qualitative form and explain how they lead to the existence of electromagnetic waves.** Solution: Maxwell's four equations (qualitative form): 1. Gauss's Law: Electric charges produce electric fields. The total electric flux through a closed surface equals the enclosed charge divided by ε₀. 2. No Magnetic Monopoles: Magnetic field lines always form closed loops. There are no isolated magnetic charges (monopoles). 3. Faraday's Law: A changing magnetic field produces an electric field. The induced electric field forms closed loops around the changing magnetic flux. 4. Ampère-Maxwell Law: An electric current produces a magnetic field. Additionally, a changing electric field also produces a magnetic field, even without a current. How they lead to EM waves: Consider an oscillating charge. By equation (1), it produces an oscillating electric field. By equation (4), this changing E-field generates a changing magnetic field. By equation (3), this changing B-field generates a changing electric field. This mutual induction creates a self-propagating disturbance—an electromagnetic wave. The speed of this wave is determined by the constants in the equations: c = 1/√(ε₀μ₀) ≈ 3 × 10⁸ m/s. Maxwell's equations thus unify electricity, magnetism, and light into a single theory. **Question 3: A microwave oven operates at a frequency of 2.45 GHz. Calculate the wavelength of the microwave radiation. If the oven heats 1 litre of water from 25°C to 75°C in 5 minutes, estimate the power of the oven (assume density of water = 1000 kg/m³, specific heat capacity of water = 4200 J/kg·°C).** Solution: Part (a): Wavelength calculation Given: f = 2.45 GHz = 2.45 × 10⁹ Hz, c = 3 × 10⁸ m/s Using c = f × λ: λ = c / f = (3 × 10⁸) / (2.45 × 10⁹) = 0.122 m ≈ 12.2 cm Part (b): Power calculation Given: Volume V = 1 L = 0.001 m³, mass m = 1000 kg/m³ × 0.001 m³ = 1 kg Temperature change: ΔT = 75 – 25 = 50°C Time: t = 5 minutes = 300 seconds Heat energy required: Q = m × c × ΔT = 1 × 4200 × 50 = 210,000 J Power: P = Q / t = 210,000 / 300 = 700 W The oven operates at approximately 700 watts. (Note: In practice, microwave ovens are ~65% efficient, so the input power would be higher.)

**Case Study: Mobile Phone Communication and Microwave Safety** Modern mobile phones use electromagnetic waves in the microwave range (typically 900 MHz to 2.6 GHz) to transmit and receive signals. A particular 4G network operates at a frequency of 1.8 GHz. Health agencies recommend that the specific absorption rate (SAR)—the power absorbed per unit mass of tissue—should not exceed 2 W/kg. A smartphone battery supplies 10 W of power to the transmitter, but only 50% of this is actually radiated; the rest is lost as heat. **Part A:** Calculate the wavelength of the 1.8 GHz signal used by this 4G network. Is this wavelength comparable to the size of human cells (~10 µm)? **Part B:** If a user holds the phone 2 cm from their ear, and the radiated power spreads over a hemispherical surface, calculate the intensity (power per unit area) at the ear. Then estimate the mass of tissue receiving this power over an area of 1 cm² and determine if the SAR limit is exceeded. **Part C:** Explain why microwaves can penetrate food and cause heating, but visible light cannot. **Complete Solution:** **Part A:** λ = c / f = (3 × 10⁸) / (1.8 × 10⁹) = 0.167 m ≈ 16.7 cm This wavelength (16.7 cm) is much larger than human cells (~10 µm = 10 × 10⁻⁶ m). Cells cannot "resonate" with wavelengths far larger than their size, so microwave effects are thermal, not resonant ionization. **Part B:** Radiated power: P_rad = 0.5 × 10 = 5 W Hemispherical surface area at r = 2 cm = 0.02 m: A = 2πr² = 2π(0.02)² ≈ 0.00251 m² Intensity: I = P_rad / A = 5 / 0.00251 ≈ 1990 W/m² For a 1 cm² area = 10⁻⁴ m²: Power absorbed: P_tissue = I × A = 1990 × 10⁻⁴ ≈ 0.199 W Assuming tissue density ≈ 1000 kg/m³ and depth of 1 cm (10⁻² m): Mass: m = 1000 × 10⁻⁴ × 10⁻² = 0.001 kg = 1 gram SAR = P_tissue / m = 0.199 / 0.001 ≈ 199 W/kg This exceeds the 2 W/kg limit significantly—but real phones are designed with antenna shielding and signal strength regulation to keep actual SAR well below limits. **Part C:** Microwaves have wavelength ~1–10 cm and can penetrate 1–2 cm into food because the wavelength is comparable to food thickness. Water molecules (dipoles) in food oscillate at the microwave frequency, generating frictional heat. Visible light has wavelength ~500 nm (0.0005 mm), far smaller than food structure. It is absorbed by pigments and scattered by particles in the first fraction of a millimetre, never reaching the interior.

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