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Electromagnetic Waves Class 12 Important Questions 2026-27 | Chapter 8

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CBSE Class 12 Physics • Chapter 8 • 2026–27

Electromagnetic Waves — Important Questions with Answers

A chapter-wise practice set covering displacement current, electromagnetic-wave characteristics, transverse nature, electromagnetic spectrum, applications, field-amplitude relations and short numericals.

Important: These are original practice questions, not official CBSE questions. Use the latest CBSE curriculum and sample paper as the final authority.

Best use: Attempt each question before opening the answer. Then revise the concept, formula and wording used in the solution.
CBSE 2026–27 scope first:

The official syllabus for Chapter 8 includes the basic idea of displacement current; electromagnetic waves and their characteristics; qualitative transverse nature; and the electromagnetic spectrum with elementary uses of radio waves, microwaves, infrared, visible, ultraviolet, X-rays and gamma rays. Chapter 8 is part of Unit V, which is grouped with Optics for 18 marks collectively; CBSE does not assign a separate fixed mark total to Chapter 8.

1. How to Prepare Chapter 8 from These Questions

  • First master displacement current and the charging-capacitor picture.
  • Then learn the field geometry and characteristics of an EM wave.
  • Memorise the radio → gamma spectrum order and connect every region with at least one elementary use.
  • Practise c = νλ and E = hν = hc/λ applications.
  • For higher-level practice, know the standard vacuum relation E0 = cB0, while keeping the current syllabus scope in mind.
What current exam-oriented practice should cover:

Recent 2026-oriented practice resources repeatedly emphasize displacement current, EM-wave characteristics, field relationships, spectrum order and uses, and short numerical applications. Current CBSE preparation should therefore combine direct recall with calculation and reasoning rather than relying on a list of memorised definitions. citeturn0search0turn0search4turn0search7

2. 1-Mark Very Short Answer Questions

Q1. What is displacement current?
Answer: Displacement current is the current-like quantity associated with a changing electric flux. It is given by Id = ε0 dΦE/dt.
Q2. What is the SI unit of displacement current?
Answer: Ampere (A), the same SI unit as ordinary current.
Q3. Can electromagnetic waves propagate through vacuum?
Answer: Yes. Electromagnetic waves do not require a material medium.
Q4. Are electromagnetic waves transverse or longitudinal?
Answer: They are transverse in the plane-wave description: E and B are perpendicular to the direction of propagation.
Q5. What is the speed of electromagnetic waves in vacuum?
Answer: Approximately 3 × 108 m/s.
Q6. Write the relation between speed, frequency and wavelength of an EM wave in vacuum.
Answer: c = νλ.
Q7. Which electromagnetic radiation has the longest wavelength among the seven listed regions?
Answer: Radio waves.
Q8. Which electromagnetic radiation has the highest frequency among the seven listed regions?
Answer: Gamma rays.
Q9. Which electromagnetic region is directly detected by the human eye?
Answer: Visible light.
Q10. Which electromagnetic radiation is commonly used in radar?
Answer: Microwaves.
Q11. Which electromagnetic radiation is commonly used for medical radiography?
Answer: X-rays.
Q12. Which region of the electromagnetic spectrum is strongly associated with thermal radiation?
Answer: Infrared radiation.
Q13. Write the photon-energy relation for electromagnetic radiation.
Answer: E = hν = hc/λ.
Q14. Arrange radio waves, infrared, ultraviolet and gamma rays in increasing frequency.
Answer: Radio waves < infrared < ultraviolet < gamma rays.
Q15. What happens to wavelength when frequency increases in vacuum?
Answer: Wavelength decreases, because c = νλ and c is fixed in vacuum.

3. 2-Mark Short Answer Questions

Q16. Why was the idea of displacement current introduced?
Answer: In a charging capacitor, conduction current flows through the connecting wire but does not cross the insulating gap. The changing electric flux in the gap is represented by displacement current, making the magnetic-field description consistent.
Q17. Differentiate between conduction current and displacement current.
Answer: Conduction current: associated with actual charge transport through a conductor. Displacement current: associated with changing electric flux. Both have the SI unit ampere.
Q18. Write the expression for displacement current and define its symbols.
Answer: The expression is Id = ε0 dΦE/dt, where ε0 is permittivity of free space and ΦE is electric flux.
Q19. Why can sunlight reach Earth through outer space?
Answer: Sunlight is electromagnetic radiation, and electromagnetic waves can propagate through vacuum without requiring a material medium.
Q20. State two important characteristics of electromagnetic waves.
Answer: They can propagate through vacuum and, for a plane wave, the electric and magnetic fields are mutually perpendicular and perpendicular to the direction of propagation.
Q21. How are electric field, magnetic field and propagation direction related in an EM wave?
Answer: For a plane electromagnetic wave, E ⟂ B, E is perpendicular to propagation direction, and B is also perpendicular to propagation direction.
Q22. Arrange the seven electromagnetic regions from longest wavelength to shortest wavelength.
Answer: Radio → Microwave → Infrared → Visible → Ultraviolet → X-rays → Gamma rays.
Q23. Arrange the seven electromagnetic regions from lowest frequency to highest frequency.
Answer: Radio → Microwave → Infrared → Visible → Ultraviolet → X-rays → Gamma rays.
Q24. Why does photon energy increase from radio waves toward gamma rays?
Answer: Photon energy is E = hν. Frequency increases from radio waves toward gamma rays, so photon energy also increases.
Q25. Why are radio waves suitable for communication?
Answer: They have relatively long wavelengths and are widely used for radio broadcasting and wireless communication systems.
Q26. Give two uses of microwaves.
Answer: Microwaves are used in radar and in communication systems such as satellite links; they are also used for microwave heating.
Q27. Give two uses or applications of infrared radiation.
Answer: Infrared is used in thermal imaging and remote-control systems; it is also associated with thermal radiation.
Q28. Give two uses or applications of ultraviolet radiation.
Answer: Ultraviolet radiation is used for fluorescence and disinfection/sterilisation.
Q29. Why is visible light called only a small part of the electromagnetic spectrum?
Answer: The human eye detects only a narrow range of electromagnetic frequencies; radio through gamma radiation extends far beyond visible frequencies.
Q30. What is the relation between wavelength and photon energy?
Answer: Since E = hc/λ, photon energy is inversely proportional to wavelength for radiation in vacuum.

4. 3-Mark Short Answer & Reasoning Questions

Q31. Explain the role of displacement current in a charging capacitor.
Answer: During charging, conduction current exists in the external wire but not across the insulating gap. The electric field between the plates changes, producing displacement current Id = ε0 dΦE/dt. For an ideal charging capacitor, the displacement-current magnitude in the gap equals the conduction-current magnitude in the wire.
Q32. A capacitor is being charged. Is there current between its plates? Explain carefully.
Answer: There is no ordinary conduction current through the insulating gap. However, the electric flux between the plates changes, so a displacement-current term exists. Thus the current description remains continuous when the displacement current is included.
Q33. Explain why electromagnetic waves are transverse.
Answer: In a plane electromagnetic wave, the electric field oscillates perpendicular to the direction of propagation, and the magnetic field also oscillates perpendicular to that direction. E and B are mutually perpendicular, so the wave is transverse.
Q34. State three characteristics that distinguish electromagnetic waves from mechanical waves.
Answer: EM waves can propagate through vacuum; they consist of coupled electric and magnetic fields; and in the plane-wave description they are transverse. Mechanical waves require a material medium.
Q35. Explain the spectrum order in terms of wavelength and frequency.
Answer: From radio to gamma rays, wavelength decreases while frequency increases. Since c = νλ in vacuum, frequency and wavelength are inversely related. Photon energy E = hν therefore also increases toward gamma rays.
Q36. A wave has frequency 6 × 108 Hz in vacuum. Find its wavelength.
Answer: Using λ = c/ν: λ = (3 × 108)/(6 × 108) = 0.5 m.
Q37. A 60 MHz electromagnetic wave travels in vacuum. Calculate its wavelength.
Answer: 60 MHz = 60 × 106 Hz. λ = c/ν = (3 × 108)/(60 × 106) = 5 m.
Q38. A radiation has wavelength 600 nm in vacuum. Find its frequency.
Answer: 600 nm = 600 × 10−9 m = 6 × 10−7 m. ν = c/λ = (3 × 108)/(6 × 10−7) = 5 × 1014 Hz.
Q39. An EM wave has electric-field amplitude E0. State the relation between E0 and B0 in vacuum.
Answer: For a plane EM wave in vacuum, E0/B0 = c, or equivalently E0 = cB0.
Q40. The frequency of an EM wave is doubled in vacuum. What happens to its wavelength and photon energy?
Answer: Since c = νλ, wavelength becomes half. Since E = hν, photon energy becomes double.
Q41. Compare a radio wave and an X-ray in terms of wavelength, frequency and photon energy.
Answer: Radio waves have much longer wavelength and much lower frequency than X-rays. Because E = hν, X-rays have much greater photon energy.
Q42. Why is gamma radiation considered more energetic than radio radiation?
Answer: Gamma rays have much higher frequency. Since photon energy is E = hν, their photon energy is much greater than that of radio waves.
Q43. A radiation has frequency 3 × 1015 Hz. Find its wavelength in vacuum and identify its approximate spectrum region.
Answer: λ = c/ν = (3 × 108)/(3 × 1015) = 1 × 10−7 m = 100 nm. This lies in the ultraviolet region.
Q44. Why should MHz, GHz and nm be converted before using c = νλ?
Answer: The relation requires compatible SI units: frequency in Hz and wavelength in metres. Prefix conversion prevents powers-of-ten errors.

5. Higher-Order & Board Practice Questions

Q45. A charging capacitor is connected to a current source. Explain why the magnetic field around the circuit should not depend on whether the surface chosen for Ampere's law cuts the wire or passes through the capacitor gap.
Answer: If the surface cuts the wire, it encloses conduction current. If it passes through the capacitor gap, there is no conduction current there, but the changing electric flux produces displacement current. Including displacement current makes the current contribution consistent for the two surfaces.
Q46. Explain how the electromagnetic spectrum is a continuous spectrum even though it is divided into named regions.
Answer: Electromagnetic radiation covers a continuous range of frequencies and wavelengths. Radio, microwave, infrared, visible, ultraviolet, X-ray and gamma are convenient named regions within that continuous range.
Q47. A student says: 'All electromagnetic waves have different speeds because they have different frequencies.' Correct the statement.
Answer: In vacuum, all electromagnetic waves have the same speed, c ≈ 3 × 108 m/s. Their frequencies and wavelengths differ, with c = νλ. In material media, propagation speed can depend on the medium and frequency.
Q48. A student says: 'Visible light is not an electromagnetic wave because it can be seen by the eye.' Is the statement correct? Explain.
Answer: No. Visible light is an electromagnetic wave. It is simply the narrow part of the electromagnetic spectrum to which the human eye is sensitive.
Q49. Which has greater wavelength: a 100 MHz radio wave or a 1 GHz microwave signal, assuming both travel in vacuum? Show the reasoning.
Answer: For fixed c, λ = c/ν. 100 MHz has the lower frequency, so it has the greater wavelength. Numerically, λ100MHz = 3 m and λ1GHz = 0.3 m.
Q50. A wave has E0 = 300 V/m in vacuum. Find B0.
Answer: Using E0 = cB0, B0 = E0/c = 300/(3 × 108) = 1 × 10−6 T.
Q51. A wave has B0 = 2 × 10−7 T in vacuum. Find E0.
Answer: Using E0 = cB0, E0 = (3 × 108)(2 × 10−7) = 60 V/m.
Q52. Explain why an electromagnetic wave can transfer energy even though there is no material medium.
Answer: Energy is carried by the coupled electric and magnetic fields of the electromagnetic wave. The fields propagate through space, so a material medium is not required for energy transport.
Q53. A radiation changes from wavelength 10−6 m to 10−8 m in vacuum. By what factor does its frequency change?
Answer: Since ν = c/λ, reducing wavelength by a factor of 100 increases frequency by a factor of 100.
Q54. Why are spectrum-use questions often asked together with ordering questions?
Answer: Because the spectrum regions differ systematically in frequency and wavelength, while their applications provide real-world identification clues. A student must connect the region, its position in the spectrum and its elementary use.
Q55. Which is more energetic per photon: ultraviolet radiation of 3 × 1015 Hz or visible radiation of 5 × 1014 Hz? By what factor?
Answer: Ultraviolet radiation. Since E = hν, the photon-energy ratio is 3 × 1015 / 5 × 1014 = 6.
Q56. A student memorises only the names of the seven spectrum regions. Why is that insufficient for board preparation?
Answer: Questions can require ordering by wavelength/frequency, identifying a region from its use, comparing photon energy, applying c = νλ, or reasoning about field characteristics. Understanding the relationships is therefore necessary.
Q57. Explain the difference between frequency and wavelength when comparing radio waves with gamma rays.
Answer: Radio waves have low frequency and long wavelength; gamma rays have high frequency and short wavelength. In vacuum they obey c = νλ, so the two quantities vary inversely.
Q58. A source emits EM radiation at 2.5 × 1014 Hz. Calculate its wavelength and state whether it lies in visible or infrared region.
Answer: λ = (3 × 108)/(2.5 × 1014) = 1.2 × 10−6 m = 1200 nm, which lies in the infrared region rather than visible light.
Q59. Why is the phrase 'highest frequency' not interchangeable with 'highest wavelength' in the electromagnetic spectrum?
Answer: Frequency and wavelength are inversely related in vacuum: c = νλ. Therefore, the highest frequency corresponds to the shortest wavelength, not the longest.

6. PYQ-Style Question Patterns to Practise

  • Displacement-current pattern: define displacement current, explain the charging-capacitor situation, or calculate it from changing electric flux.
  • Field-amplitude pattern: use E0 = cB0 for a wave in vacuum.
  • Spectrum pattern: identify a region from its use, arrange regions by frequency/wavelength, or compare photon energy.
  • c = νλ numerical: calculate wavelength from frequency or frequency from wavelength, with correct SI conversion.
  • Conceptual correction: identify and correct statements about medium, wave speed, transverse nature, frequency and wavelength.
  • Mixed application: combine a formula with a real-world use or a spectrum comparison.

These are practice patterns, not claims that any particular question will appear in the board examination. Recent PYQ collections show recurring coverage of displacement current, transverse nature, spectrum and field-amplitude relationships. citeturn0search7turn0search12

6. High-Yield Chapter 8 Topic Checklist

  • ☐ Displacement current and its formula
  • ☐ Charging capacitor: conduction current vs displacement current
  • ☐ Electromagnetic-wave characteristics
  • ☐ Transverse nature and E–B–propagation geometry
  • ☐ Speed of EM waves in vacuum
  • ☐ c = νλ and unit conversion
  • ☐ E = hν = hc/λ
  • ☐ E0 = cB0 for vacuum practice
  • ☐ Spectrum order from radio to gamma
  • ☐ Elementary uses of all seven spectrum regions
  • ☐ Application and reasoning questions

8. Quick Answers: Most-Asked Chapter 8 Questions

What is displacement current? It is the current-like quantity associated with changing electric flux: Id = ε0 dΦE/dt.

What is the order of the electromagnetic spectrum? Radio → Microwave → Infrared → Visible → Ultraviolet → X-ray → Gamma.

Which EM wave has the longest wavelength? Radio waves.

Which EM wave has the highest frequency? Gamma rays.

Do electromagnetic waves require a material medium? No. They can propagate through vacuum.

What is the relation between E0 and B0 in vacuum? E0 = cB0.

9. Exam-Smart Scope Note

Do not confuse current syllabus scope with everything available in older Chapter 8 question banks. Online resources may include energy density, radiation pressure, detailed wavelength ranges or advanced Maxwell-equation treatment. Use such material only as enrichment unless the latest CBSE/NCERT material or your school requires it.

The official 2026–27 Physics paper design is 70 marks for 3 hours, with approximately 38% Remembering/Understanding, 32% Applying and 30% Analysing/Evaluating/Creating. This is the overall paper template, not a Chapter 8-specific mark allocation.

10. Chapter Navigation

Chapter 8 Complete Notes — Electromagnetic Waves

Chapter 7 Notes — Alternating Current

Chapter 6 Notes — Electromagnetic Induction

11. Official References

Learn Revise Hub: These are original chapter-wise practice questions prepared for CBSE Class 12 Physics 2026–27. They are not official CBSE questions unless explicitly identified as such.

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