LEARN REVISE HUB · CBSE STUDY RESOURCE
Electromagnetic Waves Class 12 Physics Case-Based Questions 2026-27 | Chapter 8
CBSE Class 12 Physics • Chapter 8 • 2026–27
Electromagnetic Waves — Case-Based Questions with Answers
Application-oriented, case study and passage-based practice built around situations, observations, data and physical reasoning. The set follows the current Chapter 8 boundary: displacement current, electromagnetic-wave characteristics, transverse nature, electromagnetic spectrum and elementary uses.
10 Case Studies40 QuestionsWith AnswersCBSE 2026–27 AlignedApplication + Analysis
What This Case-Based Set Covers
Concept ApplicationDisplacement current, EM-wave properties and propagation.
Data & ReasoningFrequency, wavelength, speed and field-amplitude relationships.
Spectrum & UsesSeven regions, ordering and elementary real-world applications.
Exam note: The official 2026–27 CBSE Physics curriculum places Chapter 8 in Unit V, with Unit V + Unit VI carrying 18 marks collectively; CBSE does not publish a separate fixed mark allocation for Chapter 8 alone. The official Class XII 2026–27 SQP/MS page is the current source for sample-paper guidance.
Syllabus discipline: The current CBSE Chapter 8 scope covers the basic idea of displacement current, electromagnetic waves and their characteristics, their transverse nature qualitatively, the electromagnetic spectrum and elementary uses. Advanced material found in some older/competitive resources is not treated as compulsory board content here.
CBSE Class 12 Physics Chapter 8 Case Study & Passage-Based Questions
01
Charging Capacitor and Displacement Current
Core concept: displacement current
A parallel-plate capacitor is connected to a time-varying source. While the capacitor is charging, conduction current flows through the connecting wires, but there is no physical flow of charge across the insulating gap between the plates. The electric field between the plates changes with time. Maxwell introduced the idea of displacement current so that the magnetic effect associated with the changing electric field could be treated consistently with the current in the circuit.
Application(a) Why is displacement current associated with the gap between the capacitor plates?
Answer: Because the electric field between the plates changes while the capacitor is charging. The changing electric flux produces displacement current even though charges do not cross the dielectric gap.
Application(b) What happens to displacement current when the electric flux through the capacitor increases more rapidly?
Answer: It increases, because Id = ε0 dΦE/dt for the standard vacuum expression.
Application(c) During charging, if the rate of change of electric flux is constant, what can be said about displacement current?
Answer: It remains constant because the rate of change of electric flux, dΦE/dt, is constant.
Application(d) State the conceptual role of displacement current in the capacitor situation.
Answer: It completes the current description in a way that maintains consistency between the changing electric field in the capacitor gap and the magnetic-field effect associated with the circuit current.
02
EM Wave in Vacuum
Core concept: characteristics and transverse nature
An electromagnetic wave travels through vacuum. At a particular instant, the electric field points along the y-axis and the magnetic field points along the z-axis. The wave travels in a direction perpendicular to both fields. Unlike mechanical waves, the wave does not require a material medium for propagation.
Application(a) In what direction does the wave propagate?
Answer: Along the direction of E × B. For E along +y and B along +z, E × B points along +x.
Application(b) Are the electric and magnetic fields parallel or perpendicular to each other?
Answer: They are mutually perpendicular.
Application(c) What does the situation show about the nature of an electromagnetic wave?
Answer: It is transverse: the electric and magnetic fields are perpendicular to the direction of propagation.
Application(d) Does the wave need air, water or another material medium to travel through vacuum?
Answer: No. Electromagnetic waves can propagate through vacuum.
03
Comparing Frequency and Wavelength
Core concept: c = νλ and propagation through media
A student compares an electromagnetic wave in vacuum with the same radiation after it enters a transparent medium. The student notices that the wave travels more slowly in the medium. The frequency is determined by the source and does not change merely because the radiation enters the new medium.
Application(a) What happens to the frequency when the radiation enters the transparent medium?
Answer: The frequency remains unchanged.
Application(b) What happens to its wavelength if its speed decreases while the source frequency stays fixed?
Answer: Its wavelength decreases because λ = v/ν and ν remains constant.
Application(c) If the speed in vacuum is c and the medium has refractive index n, what is the speed in the medium?
Answer: v = c/n.
Application(d) A student says that the frequency must decrease because the speed decreases. Is the statement correct?
Answer: No. The frequency is fixed by the source; the wavelength changes to accommodate the new speed.
04
EM Spectrum at a Hospital
Core concept: spectrum regions and uses
A hospital uses different parts of the electromagnetic spectrum for different purposes. X-rays are used to obtain images of internal structures. Infrared radiation is associated with thermal effects and is used in some sensing and therapeutic contexts. Ultraviolet radiation can be used for sterilisation, while certain gamma-ray applications are used in medical treatment.
Application(a) Which region is commonly used for medical imaging of bones?
Answer: X-rays.
Application(b) Which region is associated with heating effects and is commonly called heat radiation?
Answer: Infrared radiation.
Application(c) Which region can be used for sterilisation because of its ability to damage microorganisms?
Answer: Ultraviolet radiation.
Application(d) Arrange X-rays, infrared, ultraviolet and gamma rays in increasing frequency.
Answer: Infrared < ultraviolet < X-rays < gamma rays.
05
Communication and Radar
Core concept: radio waves and microwaves
A communication system uses electromagnetic radiation to transfer information over long distances. Another system sends radiation toward an object and analyses the reflected signal to determine information about its position. Different frequency regions are selected because their propagation and interaction characteristics make them suitable for different applications.
Application(a) Which broad region is widely associated with radio communication?
Answer: Radio waves.
Application(b) Which region is commonly used in many radar systems?
Answer: Microwaves.
Application(c) Are radio waves and microwaves both electromagnetic waves?
Answer: Yes. Both are regions of the electromagnetic spectrum.
Application(d) If two electromagnetic radiations travel in vacuum, does the higher-frequency one travel faster?
Answer: No. In vacuum, all electromagnetic waves travel at the same speed c; their frequencies and wavelengths differ.
06
Reading the Spectrum from Wavelength
Core concept: frequency, wavelength and photon energy
A laboratory source produces three electromagnetic radiations. Radiation A has a wavelength of 600 nm, radiation B has a wavelength of 600 pm, and radiation C has a wavelength of 60 μm. The students compare their frequencies and photon energies using ν = c/λ and E = hν.
Application(a) Which radiation has the highest frequency?
Answer: Radiation B, because it has the shortest wavelength.
Application(b) Which radiation has the lowest photon energy?
Answer: Radiation C, because it has the lowest frequency; using E = hν = hc/λ, lower frequency means lower photon energy. This is quantitative reinforcement rather than a separate Chapter 8 syllabus heading.
Application(c) As a quantitative reinforcement, if wavelength is reduced, what happens to photon energy?
Answer: Photon energy increases because E = hc/λ. This relationship is useful for interpreting radiation-energy questions, but it should not replace the current Chapter 8 core topics.
Application(d) State the relation connecting frequency and wavelength in vacuum.
Answer: c = νλ.
07
Electric and Magnetic Fields in Phase
Core concept: field relationship in an EM wave
A detector receives a plane electromagnetic wave travelling through vacuum. Measurements show that the electric field and magnetic field oscillate periodically. The maximum electric-field amplitude is E0 and the maximum magnetic-field amplitude is B0. The student is asked to connect the two fields with the speed of light.
Application(a) What is the phase relationship between E and B in a plane electromagnetic wave?
Answer: They are in phase: their maxima and minima occur simultaneously at corresponding positions.
Application(b) State the relation between E0, B0 and c in vacuum.
Answer: E0/B0 = c, or E0 = cB0.
Application(c) If B0 is doubled while the wave remains in vacuum, what happens to E0?
Answer: E0 also doubles because E0 = cB0.
Application(d) Does the relation mean that E and B point in the same direction?
Answer: No. Their amplitudes are related, but the fields remain perpendicular to each other.
08
Identifying Radiation from Its Use
Core concept: spectrum applications
A science museum displays four devices: a TV remote control, an airport communication/radar system, a medical radiography machine, and a device used for detecting thermal radiation. Students must identify the relevant electromagnetic-wave regions and then compare their frequencies.
Application(a) Which radiation is commonly used in TV remote controls?
Answer: Infrared radiation.
Application(b) Which radiation is commonly associated with radar?
Answer: Microwaves.
Application(c) Which radiation is used in radiography?
Answer: X-rays.
Application(d) Among infrared, microwaves and X-rays, which has the highest frequency?
Answer: X-rays.
09
Displacement Current as a Changing-Field Effect
Core concept: qualitative reasoning
Two situations are compared. In Situation I, the electric field in a region is constant with time. In Situation II, the electric field changes with time because a capacitor is being charged. The students discuss whether displacement current should be associated with the two situations.
Application(a) In which situation is displacement current associated with the changing electric field?
Answer: Situation II.
Application(b) What quantity changes in the expression for displacement current?
Answer: Electric flux changes with time; for the standard expression, Id = ε0 dΦE/dt.
Application(c) If dΦE/dt becomes zero, what is the displacement current?
Answer: It is zero.
Application(d) Why is this idea important in electromagnetic theory?
Answer: It accounts for the magnetic effect associated with a changing electric field and resolves the current-description issue in situations such as a charging capacitor.
10
Choosing the Correct Spectrum Region
Core concept: spectrum order, frequency and wavelength
A teacher gives four clues: (1) radiation with the longest wavelength in the standard electromagnetic spectrum, (2) radiation commonly associated with thermal imaging, (3) radiation with very high frequency used in some cancer-treatment applications, and (4) radiation used for medical imaging of bones. Students must identify each region and justify the ordering using frequency and wavelength.
Application(a) Which region has the longest wavelength among the seven standard regions?
Answer: Radio waves.
Application(b) Which region is associated with thermal imaging?
Answer: Infrared radiation.
Application(c) Which region is associated with some radiotherapy applications?
Answer: Gamma rays.
Application(d) Give the correct increasing-frequency order of the seven standard regions.
Answer: Radio waves < microwaves < infrared < visible < ultraviolet < X-rays < gamma rays.
How to Solve a CBSE-Style Case Study
- Identify the concept: Decide whether the case is testing displacement current, wave properties, propagation, spectrum order or an application.
- Extract the given information: Look for direction, frequency, wavelength, medium, field amplitude or the practical use described.
- Apply the shortest valid relation: For example, c = νλ in vacuum, v = c/n in a medium, and E0/B0 = c in vacuum.
- Check the syllabus boundary: Do not import advanced Maxwell-equation mathematics or unrelated optics content simply because a source mentions it.
- Answer precisely: State the concept first, then give the reason or calculation needed by the question.
Quick Answer & Revision Bank
| Concept | Remember |
|---|---|
| Displacement current | Id = ε0 dΦE/dt in vacuum; associated with a changing electric flux. |
| Nature of EM waves | Transverse; E and B are mutually perpendicular and both are perpendicular to propagation. |
| Direction of propagation | Along E × B. |
| Speed in vacuum | All electromagnetic waves travel at c in vacuum. |
| Wave relation | c = νλ in vacuum. |
| Field-amplitude relation | E0/B0 = c in vacuum. |
| Photon energy | E = hν = hc/λ. |
| Spectrum order | Radio → microwave → infrared → visible → ultraviolet → X-ray → gamma. |
Frequently Asked Questions
Are these official CBSE case-based questions?
No. They are original practice questions designed around the current syllabus and CBSE-style application/analysis. Use official CBSE SQPs, marking schemes and question papers for official examples.
Which Chapter 8 concepts are most useful for case-based practice?
Displacement current, transverse nature of electromagnetic waves, E–B relationships, propagation, frequency/wavelength reasoning, spectrum order and elementary applications.
Can electromagnetic waves travel through vacuum?
Yes. They do not require a material medium.
What is the frequency order of the electromagnetic spectrum?
Radio waves → microwaves → infrared → visible → ultraviolet → X-rays → gamma rays.
Continue Chapter 8 Preparation
Source & Content Discipline
- CBSE 2026–27 Physics curriculum: the primary authority for Chapter 8 scope and the unit-level marks structure.
- CBSE Class XII 2026–27 SQP/MS: the current official source for sample-paper and marking-scheme guidance.
- NCERT Physics Part-I, Chapter 8: used as the textbook foundation for displacement current, electromagnetic waves and the spectrum.
- Competitor/search-intent research: current and established educational resources were reviewed for recurring queries such as “case study questions”, “passage-based questions”, displacement current, spectrum order and applications. These sources inform coverage and wording only; they do not override CBSE/NCERT scope.
- The questions on this page are original practice material and should not be described as predicted or guaranteed board questions.
Link audit: Internal links on this page point only to the four already-published Chapter 8 resources (Notes, Important Questions, MCQs and Numericals) plus published Chapter 6 and Chapter 7 Notes. No future Chapter 8 Assertion–Reason, PYQ, Formula Sheet or Chapter Test URL is used as a placeholder. Official links point to CBSE curriculum, CBSE 2026–27 SQP/MS and NCERT Physics Part-I.
Official CBSE curriculum: Physics 2026–27 Curriculum · Official SQP/MS page: Class XII 2026–27 SQP & MS · NCERT Physics Part-I: NCERT Physics Part-I PDF
Comments
Post a Comment